Keep in mind what Jesus had said earlier...

Let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven. (Matthew 5:16-note)
 

From this verse and Mt 5:20, it follows that the 6 segments in Mt 5:21-48 serve at least a twofold purpose: (1) To teach what righteousness looks like that surpasses the Scribes and Pharisees and (2) To describe the righteousness when lived out in the power of the Spirit gives a proper opinion of our heavenly Father. Keep this command in (Mt 5:16-note) in mind as you meditate on each segment. Ask yourself in each case "Do my attitudes and actions in this area of my life give others I meet a proper opinion of my Father Who is in heaven?" We are called to be perfect as our heavenly Father is perfect that the world might see His glory in and through us. This is a "heavy" calling but best of all it is a holy calling which He has enabled citizens under the New Covenant to carry out by virtue of their new hearts upon which the Law is written and the indwelling Spirit Who causes us to walk according to His statutes.

It is interesting that Jesus begins with an emphasis on the sanctity of every human life and the call on Kingdom citizens to do all they can to honor that sanctity.

So what is the problem with what the Scribes and Pharisees had taught? The problem is that they were not good inductive Bible students and because of inadequate observation and reliance on the "commentaries" (what the rabbis had taught as "tradition"), they arrived at an inaccurate interpretation of the Law and consequently, and most importantly, they prescribed inappropriate application based on their willfully inept analysis of the OT Scriptures. Their misapplication of the Law led to a liberal attitude toward murder, adultery, divorce, vows, retaliation and love. Therefore Jesus calls His listeners and we the readers of His sermon to exhibit allegiance to a higher standard, a standard of righteousness that far surpasses that of the Scribes and Pharisees and which shines a beacon of supernatural light that points the lost to the great and mighty supernatural Father of lights, the Heavenly Father. (Mt 5:16-note)

In Mt 5:48 (note) Jesus climaxes this section with the incredible statement to be perfect as your heavenly Father is perfect. Citizens of the Kingdom of Heaven are called to measure themselves not by others but by their Father Who art in heaven. The “sons” are to be perfect, as their heavenly Father is perfect. and as Jesus proceeds to explain, that perfection is absolute perfection and includes our words (notes Matthew 5:33-35, 5:36-37), our responses to injuries (Mt 5:38, 39, 40, 41, 42-notes Mt 5:38-39, 40-42), and our dealings with our enemies (Mt 5:43, 44, 45, 46, 47, 48-notes Mt 5:43-45, 46-48).

At the time of Jesus, the Jews were following the letter of the Law but not the spirit of the Law. In Matthew 23 Jesus is speaks 7 woes to the Scribes and Pharisees, those whose righteousness must be exceeded to enter heaven. One of those woes summarizes what the religious leaders were doing and teaching the people to do...

"Woe to you, scribes and Pharisees, hypocrites! For you tithe mint and dill and cummin, and have neglected the weightier provisions of the law: justice and mercy and faithfulness; but these are the things you should have done without neglecting the others. You blind guides, who strain out a gnat and swallow a camel! (Mt 23:23)

They were so worried about the little details of the letter of the Law but they were missing the heart of the Law and that is what Jesus explains with 6 illustrations in Mt 5:21-48.

Sinclair Ferguson has some interesting introductory comments to Mt 5:21-48, reminding us that...

Jesus came to fulfill the law, not to destroy it. In fact, as we have seen, Jesus teaches that the Law of God is an essential diagnostic tool. Whether we break it or keep it, and whether we encourage others to break it or keep it, is an indication of our true spiritual condition. It is the standard for evaluation in the kingdom of God (Matt. 5:19), but not the standard for entrance into the kingdom. Rather than dispensing with righteousness, Jesus tell his disciples that unless their righteousness surpasses that of the Pharisees and the teachers of the law, they will certainly not enter the kingdom of heaven...It is no accident that these contrasts ("you have heard...but I say") are set in the context of Jesus as the fulfilment of the Exodus: He is the Son God has called out of Egypt (Mt 2:15); He passes through the waters in His baptism (Mt 3:13, 14, 15, 16, 17); He is tested in the wilderness (Mt 4:1-11); He expounds the law of God in the mountainous region (Mt 5:1). Rather than displace the teaching God had given through Moses, Jesus' whole ministry is identified by Matthew as the fulfilment of God's teaching. (Ferguson, Sinclair: Sermon on the Mount :Banner of Truth)

Charles Simeon writes...

GENERAL statements, and general invectives, rarely carry any conviction to the mind: they must be supported by an induction of particulars, before they can produce any material effect. Assertions without proofs will be taken for calumny; but, when supported by fact, they will bear down all opposition. The assertions of our blessed Lord, indeed, needed no confirmation; because “he knew what was in man;” and because his miracles were a sufficient attestation to his word. Yet even He condescended to substantiate his accusations by appeals to fact. He had intimated that the Scribes and Pharisees both did and taught many things contrary to his revealed will: and he had declared, that unless we have a better righteousness than theirs, we cannot enter into the kingdom of heaven. To evince the truth of his charge, and of the declaration founded upon it, he shows, that they had grossly perverted the sixth commandment: which on that account he proceeds to explain. (Read the entire sermon - Matthew 5:21,22 Christ's Exposition of the Sixth Commandment - Goto Page 113)

You have heard suggests that the multitudes listening had for the most part not studied the Law of Moses for themselves but had only heard the teaching on the Law, most likely from the scribes and Pharisees.

Ancients (744) (archaios [word study] from arche = beginning) means old, expressing that which was from the beginning in contrast to palaios [word study], old, as having existed a long period of time.  Archaios reaches back to a beginning, whenever that beginning may have been.

TDNT says archaios...

means “from the beginning,” then “past” or “old,” often with a reference to origins and with something of the dignity of “ancient.” In the LXX it can sometimes have the sense of pre-temporal, as in Is 37:26....in 2Co 5:17 pre-resurrection (Ed: Before our "co-crucifixion" and "co-resurrection" with Christ) religious relations and attitudes are in view.

Archaios - 11x in 11v in the NAS - Mt 5:21-note, Mt 5:33-note; Lk 9:8, 19; Acts 15:7, 21; 21:16; 2Cor 5:17; 2Pe 2:5-note; Rev 12:9-note Re 20:2-note

Murder (5407) (phoneuo) (Dictionary Article) means to kill a man unjustly. Webster (modern version) says that murder is the crime of unlawfully killing a person especially with malice aforethought. (See topic Murder) In the OT passages (Exodus 20:13; Deuteronomy 5:17) the Hebrew word for "murder" refers to pre-meditated, deliberate, intentional murder not accidental killing.

The 1828 edition of Webster's Dictionary defines murder as "he act of unlawfully killing a human being with premeditated malice, by a person of sound mind. To constitute murder in law, the person killing another must be of sound mind or in possession of his reason, and the act must be done with malice prepense, aforethought or premeditated; but malice may be implied, as well as express."

Moses records God's decree after the flood...

And surely I will require your lifeblood; from every beast I will require it. And from every man, from every man's brother I will require the life of man. Whoever sheds man's blood, By man his blood shall be shed, For in the image of God He made man. Genesis 9:5,6

Ryrie comments that...

Homicide (which in a sense is always fratricide [killing a "brother"], v. 5) demands a punishment that matches the crime. The justification for capital punishment, here established, is the nobility of human life, which is made in the image of God. Thus murder shows contempt for God as well as for one's fellow man. See Ro 13:4-note, where government is given the power of life or death. (The Ryrie Study Bible: New American Standard Translation: 1995. Moody Publishers)

AND 'WHOEVER COMMITS MURDER SHALL BE LIABLE TO THE COURT (Exodus 21:12, 13, 14; Numbers 35:12,16, 17, 18, 19, 20, 21,30, 31, 32, 33, 34; Deuteronomy 21:7, 8, 9; 1Kings 2:5,6,31,32)

Liable (1777) (enochos from enécho = hold in or to be ensnared) pertains to being guilty for having done wrong and thus deserving some particular penalty. It is primarily a legal term.

Enochos - 10x in 8v - Matt 5:21, 22; 26:66; Mark 3:29; 14:64; 1Cor 11:27; Heb 2:15; Jas 2:10. The NAS renders it as deserving(2), guilty(6), liable to(1), subject to(1).

The Old Testament did provide a way of escape from the death penalty if the death was accidental. Moses records that...

BUT I SAY TO YOU THAT EVERYONE WHO IS ANGRY WITH HIS BROTHER SHALL BE GUILTY BEFORE THE COURT (Mt 5:28,34,44; 3:17; 17:5; Deuteronomy 18:18,19; Acts 3:20, 21, 22, 23; 7:37; Hebrews 5:9; 12:25) (Genesis 4:5,6; 37:4,8; 1Samuel 17:27,28; 18:8,9; 20:30, 31, 32, 33; 22:12-23; 1Kings 21:4; 2Chronicles 16:10; Esther 3:5,6; Psalms 37:8; Daniel 2:12,13; 3:13,19; Ephesians 4:26,27) (Mt 5:23,24; 18:21,35; Deuteronomy 15:11; Nehemiah 5:8; Obadiah 1:10,12; Romans 12:10; 1Corinthians 6:6; 1Thessalonians 4:6; 1John 2:9; 3:10,14,15; 4:20,21; 5:16) (Psalms 7:4; 25:3; 35:19; 69:4; 109:3; Lamentations 3:52; John 15:25)

Spurgeon comments...

Oh, what divine dignity there is in this majestic Person whose ipse dixit is to shift all the sayings of the ages! He claims authority to speak, even though he should contradict all the Rabbis and all the learned men that went before him: “I say unto you,”

But you are saying "I have never murdered anyone". Jesus says "Wrong. If you've hated you are guilty."

The OT law dealt only with outward actions, but citizens of the kingdom of heaven must beware of sinful inward attitudes. In fact, the attitudes Jesus calls for can only be fulfilled by those with a New Heart and His Spirit, for otherwise they are an utterly impossible standard which climaxes in the highest of all impossible standards to "Be perfect as your heavenly Father is perfect." Only citizens of the Kingdom of heaven, genuine believers, can obey Jesus' instructions and commands, doing so not because of outward constraints (not "under Law") but because of an inward life ("under grace") enabled by the indwelling Spirit of the Living God. You might argue that the Holy Spirit is not even mentioned in the Sermon on the Mount and you would be correct. But from the character of the life Jesus calls for, it is unarguably clear that one cannot practice such behavior apart from the Spirit’s empowerment (see Romans 8, notes begin on Romans 8:1).

The phrase "but I say" is not to suggest that the Law was merely external but was always intended to be kept from the heart. And so we see this sentiment in the first and great commandment (Mark 12:30) in Deuteronomy where God commands Israel

And you shall love the LORD your God with all your heart and with all your soul and with all your might. And these words, which I am commanding you today, shall be on your heart (Deut 6:5, 6)

This call in the Old Testament to love is the essence of fulfillment of the Law as Paul explains in Romans...

Love does no wrong to a neighbor; love therefore is the fulfillment (fullness, with emphasis upon completeness) of the law. (see note Romans 13:10)

The point is that Jesus was not saying anything new or different than had already been plainly stated in the Law. He is in no way diminishing the Law. He is showing that the keeping of God's Law has always been an issue of a heart out of which flows that person's loving obedience for God and his fellow man.

"But I say" uses both the first person singular of the verb for say (lego) and the first person singular pronoun (ego) "I"  for emphasis. One might paraphrase to bring out the emphatic character as "I myself say." The point is that He emphasizing His authority as on a plain with the Law and is not merely speaking content. Stated another way, "But I say" is a way Jesus equated His Word with the authority of Scripture and thus with the authority of God, which is valid in view of the fact hat Jesus is "the Word" (John 1:1). In fact, indirectly by introducing the sentence in this manner, Jesus was underscoring His deity.

Indeed at the end of the Sermon Matthew records...

The result was that when Jesus had finished these words, the multitudes were amazed at His teaching for He was teaching them as one having authority, and not as their scribes. (Mt 7:28, 29-notes)

Jesus did not rely on the words of other men as did the scribes and Pharisees. Jesus is not speaking words that contradict or conflict with the the Law and the Prophets, but fulfilling those words, giving them the true meaning God had intended. Similar Jesus is not altering, diminishing or depriving even the least of the commandments of their authority (Ro 5:19 note) Jesus is correcting the perversion of the Law by the scribes and Pharisees who had devised 613 manmade rules that only tied heavy burdens on the people who were told that "Keep all 613 prohibitions and commandments and you will be pleasing to Him and satisfy His demands for righteousness." The problem of course was that their external rules left the heart unchanged, still more deceitful than all else and desperately sick (Jer 17:9, cf Ge 8:21).

The KJV has the phrase "without a cause", a phrase not found in most of the modern versions

Angry (3710) (orgizo from orge = wrath) means to be or become angry, furious, enraged or provoked and is used in the NT of human and satanic anger. (See Topic Anger) Orgizo describes a brooding, simmering anger that is nurtured and not allowed to die. It is seen in the holding of a grudge, in the smoldering bitterness that refuses to forgive. It is the anger that cherishes resentment and does not want reconciliation.

Orgizo - 8x in 8v - Matt 5:22; 18:34; 22:7; Luke 14:21; 15:28; Eph 4:26; Rev 11:18; 12:17. The NAS renders it as angry(4), enraged(3), moved with anger(1).

The Scribes and Pharisees taught that anything short or murder might be allowed. Jesus brings the Law back to the matters of the heart teaching that the danger of anger was that it was expressive of a murderous intent in the heart and such a one was in danger of the judgment.  Jesus says that anger merits execution, because the fruit of anger is murder.

Anger is what we feel when we believe that we have been wronged by someone. It springs from resentment over an offense and contempt for the individual. It results in an adversarial relationship that has the potential to disrupt fellowship and worship alike.

One is reminded of God's warning to Cain about his heart attitude (it was not Cain's offering which was the problem but it was the attitude of his heart!) and how when it was left unrestrained, "pounced" on him and led to murder of Abel...

Then the LORD said to Cain, "Why are you angry? (Hebrew charah =  to glow or grow warn, to blaze up and be hot or burn, be kindled, be incensed - a verb perfectly picturing this emotion "blazing up" into a full blown fire) And why has your countenance fallen (this suggests that anger hid itself in Cain’s eyes and with a fallen countenance he avoided looking anyone in the eye preventing others from seeing through the eye gate into his heart!)? If you do well, will not your countenance be lifted up? And if you do not do well, sin (this pictures Sin as an evil king that wants to reign in our mortal bodies, exactly what Paul admonished Roman saints about in Ro 6:12, 13, 14- see note) is crouching (picture of a stealthy panther lurking, ready to spring) at the door; and its desire is for you, but you must master it (If this is God's desire then we know that He made provision for Cain to obey God's will for God's commandments always include His enablements)." And Cain told Abel his brother. And it came about when they were in the field, that Cain rose up against Abel his brother and killed him. (Genesis 4:6, 7, 8)

Warren Wiersbe explains that

Anger is an emotional arousal caused by something that displeases us. In itself, anger is not a sin, because even God can be angry (Dt 9:8, 20; Ps 2:12). The Bible often speaks of anger “being kindled” (Ge 30:2; Dt 6:15), as though anger can be compared to fire. It is difficult for us to practice a truly holy anger or righteous indignation because our emotions are tainted by sin, and we do not have the same knowledge that God has in all matters. God sees everything clearly and knows everything completely, and we do not. The NT principle seems to be that the believer should be angry at sin but loving toward people. The fire of anger, if not quenched by loving forgiveness, will spread and defile and destroy the work of God (Ed note: and the testimony of God's people). According to Jesus, anger is the first step toward murder (Mt 5:21, 22, 23, 24, 25, 26), because anger gives the devil a foothold in our lives, and Satan is a murderer (Jn 8:44). Satan hates God and God’s people, and when he finds a believer with the sparks of anger in his heart, he fans those sparks, adds fuel to the fire, and does a great deal of damage to God’s people and God’s church. Both lying and anger “give peace to the devil” (Eph 4:27-note).

Horace wrote that “Anger is momentary insanity.” But that is not to say as do our law courts today that the angry person is not responsible because clearly they are!

John writes that...

The one who says he is in the light and yet hates his brother is in the darkness until now. (1John 2:9)

Everyone who hates his brother is a murderer; and you know that no murderer has eternal life abiding in him. (1John 3:15)

If someone says, "I love God," and hates his brother, he is a liar (he is not a believer); for the one who does not love his brother whom he has seen, cannot love God whom he has not seen. (1John 4:20)

Paul exhorts citizens of the Kingdom of heaven to give a proper opinion of their heavenly Father and enabled by the Holy Spirit to...

Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice. And be kind to one another, tender-hearted, forgiving each other, just as God in Christ also has forgiven you. (Eph 4:26, 27-notes)

Peter adds...

To sum up, let all be harmonious, sympathetic, brotherly, kindhearted, and humble in spirit not returning evil for evil, or insult for insult, but giving a blessing instead; for you were called for the very purpose that you might inherit a blessing. (1Pe 2:8, 9-notes)

AND WHOEVER SAYS TO HIS BROTHER, (RACA) 'YOU GOOD-FOR-NOTHING,' SHALL BE GUILTY BEFORE THE SUPREME COURT (SANHEDRIN) (2Sa 6:20; James 2:20) (Mt 10:17; 26:59; Mark 14:55; 15:1; John 11:47; Acts 5:27)

Good for nothing (see raca or raca - source #2) (4469) (raca an Aramaic word equivalent to Hebrew rebq = empty one, worthless, term of utter vilification) refers to a vain or worthless fellow and was used as a term of contempt used by the Jews in the time of Christ. English synonyms with a similar idea include nitwit, blockhead, numbskull, bonehead or brainless idiot.  "Raca" expressed contempt for an individual's intelligence.

Supreme court (4892) (sunedrion from sun = together + hedraios = sedentary, as one seated in a chair - see Sanhedrin) describes an assembly or council and in the present context describes the Jewish Sanhedrim, the council of the seventy (not counting the high priest) who tried the most serious offenses and pronounced the severest penalties, including death by stoning (see Acts 6:12-7:60) (See ISBE article on Sanhedrin) The Sanhedrin was formed in imitation of the seventy elders appointed by Moses (Nu11:16ff.) The members were selected from the chief priests, former high priests, and the chief priests or heads of the twenty–four courses or divisions, elders, and scribes or lawyers.

Sunedrion -22x in 22v-  Matt 5:22; 10:17; 26:59; Mark 13:9; 14:55; 15:1; Luke 22:66; John 11:47; Acts 4:15; 5:21, 27, 34, 41; 6:12, 15; 22:30; 23:1, 6, 15, 20, 28; 24:20. The NAS renders it as Council(m)(17), council(2), courts(m)(2), supreme court(m)(1).

MacArthur adds that raca...

has no exact modern equivalent. Therefore in most Bible versions, as here, it is simply transliterated. A term of malicious abuse, derision, and slander, it has been variously rendered as brainless idiot, worthless fellow, silly fool, empty head, blockhead, and the like. It was a word of arrogant contempt. (MacArthur, J: Matthew 1-7 Macarthur New Testament Commentary Chicago: Moody Press)

Why is simply saying "raca" or "you fool" a sin that makes one guilty enough to even go to hell? The answer in short is that every man is made in the image of God and God does not say "raca" or "you fool". Instead what God does is say I love you and to show you how much I sent my only begotten Son (John 3:16). Paul also explains how God sees us and how he responds in light of what He sees (remember we are to be perfect as our heavenly Father is perfect!)  writing that...

while we were still helpless, at the right time Christ died for the ungodly. For one will hardly die for a righteous man; though perhaps for the good man someone would dare even to die. But God demonstrates His own love toward us, in that while we were yet sinners, Christ died for us. Much more then, having now been justified by His blood, we shall be saved from the wrath of God through Him. For if while we were enemies, we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life.  (See notes Romans 5:6; 5:7; 5:8; 5:9; 5:10)

Jesus is saying that in effect our "cutting words" carry out the "assassination" of the person using the weapon of the tongue and those words that proceed from a heart filled with animosity, enmity or anger. When you call another man or woman made in the image of God, you are in effect taking the place of God and when you say "raca" or "you fool" you are holding them with lower esteem than God Himself does and thus it is a grievous sin. Isn't that what one "says" when they murder another person? They are saying in essence that "Your life is worth less to me than it is to God!" And thus the murderer "takes the place" of God.

Jesus exposition helps one understand Solomon's teaching that...

Death and life are in the power of the tongue, and those who love it will eat its fruit. (Proverbs 18:21)

AND WHOEVER SAYS, 'YOU FOOL,' SHALL BE GUILTY ENOUGH TO GO INTO THE FIERY HELL (GEHENNA) (Psalms 14:1; 49:10; 92:6; Proverbs 14:16; 18:6; Jeremiah 17:11) (Mt 5:29,30; 10:28; 18:8,9; 25:41; Mark 9:47; Luke 12:5; 16:23,24; Revelation 20:14)

Spurgeon comments that...

Christ here shows us that the commandment, “Thou shalt not kill,” deals with anger, with angry words, with words of cursing, with words of derision, for all these are killing things, hurting and wounding things, and the passion of anger is forbidden under the command, “Thou shalt not kill.” Men have not thought so, but it really is so, for he who is angry with his brother is a murderer; there is the spirit, the essence of that which leads to murder in the passion which breeds malice and revenge. The law is spiritual; it touches the emotions, the thoughts, the desires, as well as the words and actions of men. If I desire ill for a man, I have within me that which would desire his death; and what is that, after all, but murder in the heart? How strict is this law, and yet how just and right!

Fool (3474) (moros) conveys the root meaning of one who is mentally dull, silly or stupid (English = “moron”). There are a number of synonyms including blockhead, bonehead, dimwit, dork, numbskull, and the list goes on and on. Jesus seems to characterize calling someone a fool as a more serious reproach than a "good for nothing"

Moros - 12x in 12v - Matt 5:22; 7:26; 23:17; 25:2f, 8; 1 Cor 1:25, 27; 3:18; 4:10; 2 Tim 2:23; Titus 3:9. The NAS renders it as fool(1), foolish(7), foolish things(1), foolishness(1), fools(2).

MacArthur notes that moros...

was sometimes used in secular Greek literature of an obstinate, godless person. It was also possibly related to the Hebrew mārâ which means “to rebel against.” To call someone You fool was to accuse them of being both stupid and godless. The three illustrations in this verse show increasing degrees of seriousness. To be angry is the basic evil behind murder; to slander a person with a term such as Raca is even more serious, because it gives expression to that anger; and to condemn a person’s character by calling him a fool is more slanderous still...Jesus’ prohibition is against slanderously calling a person a fool out of anger and hatred. Such an expression of malicious animosity is tantamount to murder and makes us guilty enough to go into the fiery hell. (MacArthur, J: Matthew 1-7 Macarthur New Testament Commentary Chicago: Moody Press)

Hell (Gehenna) (1067) (geenna from  Hebrew gay = valley + Hinnom = a deep narrow ravine south of Jerusalem once associated with the pagan god Moloch and his disgusting rite of infant sacrifices [cp modern practice of abortion!], 2Kings 23:10; 2Chr 28:3; 33:6; Jer 7:31, 19:5, 6, 32:35; Ezekiel 16:20; 23:37 clearly prohibited by God in Lev 18:21, 20:2, 3, 4, 5) is literally the valley of Hinnom, the valley where the filth and dead animals of the city were cast out and burned and where there were trash fires and perpetually burning rubbish,  all a fit symbol of the future home of all unrepentant, unregenerate wicked men and women. It was a foul, forbidding place where the fire, smoke, and stench never ceased. It is thus fitting that gehenna is where sin and unrepentant sinners will one day find it's "resting place".

Gehenna - 12x in 12v - Mt 5:22, 29f; 10:28; 18:9; 23:15, 33; Mk 9:43, 45, 47; Lk 12:5; Jas 3:6

Both Raca and fool broke the heart of the law against murder, one by slaying a person's intelligence and the other his character.

It is interesting that Jehovah does have the right to use moros for He knows the heart and thus He declared to treacherous (adulterous) Jerusalem and Judah...

'Hear this, O foolish and senseless people, Who have eyes, but see not; Who have ears, but hear not." (Jer 5:21)

Thus There are times when the Scripture does consider men as fools. Jesus said so Himself, declaring

"You fools and blind men; which is more important, the gold, or the temple that sanctified the gold? (Mt 23:17)

God addressed the rich man who had laid up for himself goods on earth but not heaven declaring...

'You fool! This very night your soul is required of you; and now who will own what you have prepared?' (Luke 12:20)

And the psalmist explains that...

The fool has said in his heart, "There is no God." They are corrupt, they have committed abominable deeds; There is no one who does good. (Ps 14:1) (See Spurgeon's note)

Solomon presents a contrast be one who fears God and the fool writing that...

 The fear of the LORD is the beginning of knowledge; Fools despise wisdom and instruction. (Proverbs 1:7)

These examples from Scripture all have in common a stubborn rebellion against God, which is not what Jesus is referring to. For a man to call another man "you fool" is in a sense to put himself in the place of God. Jesus is specifically speaking against the deliberate belittling of someone's person because of the animosity and hatred of our own heart. That He says is as if we had murdered them!

Have you ever been guilty of "murder"?

If we are honest, most of would have to say we are guilty of his crime against our fellow man. Jesus' words give us pause to consider carefully what we speak with our tongues. Our words are generally an accurate barometer of our true spiritual condition. What is your "barometric reading" beloved? It follows that it is of little surprise that the  New Testament repeatedly  presents such clear teaching regarding the danger of the tongue...

"You brood of vipers, how can you, being evil, speak what is good? For the mouth speaks out of that which fills the heart. The good man out of his good treasure brings forth what is good; and the evil man out of his evil treasure brings forth what is evil.  And I say to you, that every careless (literally "not working" - describes a  word that produces no good effect, and is not calculated to produce any) word that men shall speak, they shall render account for it in the day of judgment. For by your words you shall be justified, and by your words you shall be condemned." (Mt 12:34-37)

"Do you not understand that everything that goes into the mouth passes into the stomach, and is eliminated?  "But the things that proceed out of the mouth come from the heart, and those defile the man.  For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. These are the things which defile the man; but to eat with unwashed hands does not defile the man." (see notes Matthew 5:17; 5:18; 5:19; 5:20)

BE ANGRY, AND YET DO NOT SIN; do not let the sun go down on your anger, and do not give the devil an opportunity... Let no unwholesome word proceed from your mouth, but only such a word as is good for edification according to the need of the moment, that it may give grace to those who hear. And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. (see notes Ephesians 4:27; 4:29; 4:30)

The International Standard Bible Encyclopedia has an interesting entry which says in part that Raca...

is generally explained as expressing contempt for a man's intellectual capacity (= "you simpleton!"), while more (translated "thou fool"), in the same verse is taken to refer to a man's moral and religious character (= "you rascal!" "you impious fellow!"). Thus we have three stages of anger, with three corresponding grades of punishment:

(1) the inner feeling of anger (orgizo), to be punished by the local or provincial court ("the judgment");

(2) anger breaking forth into an expression of scorn (Raca), to be punished by the Sanhedrin ("the council");

(3) anger culminating in abusive and defamatory language (More), to be punished by the fire of Gehenna. (ISBE)

One should be careful not to misinterpret Jesus as declaring that it makes no difference whether we kill with words like gossip or stab with literal knives of cold steel. What Jesus is teaching is that both activities reveal the same animosity of one's heart to his or her neighbour.

Sinclair Ferguson wisely concludes that...

Jesus recognized that we cannot be trusted in our judgment of the seriousness of careless speech. We treat the damage we do with our lips very lightly because we do not see the corpses we leave behind. That is why Jesus invades our moral slumber by telling us how serious this is in the sight of God. He uses language we readily understand: anger incurs judgment; using terms of con­tempt (like raca) is worthy of condemnation by the highest court (Mt 5:22-note); calling someone a fool fits us for hell. Jesus is probably not placing these sins on a scale of seriousness in the kingdom of God; He is simply stressing vividly that they are far more serious than most of us assume. In fact, our insensitivity to their real seriousness is indicative of the dullness of our spiritual senses. (Ferguson, Sinclair: Sermon on the Mount :Banner of Truth)

Freeman's Manners and Customs of the Bible has the following summary of Mt 5:22:...

The Jewish teachers had heretofore taught that nothing except actual murder was forbidden by the sixth commandment. Thus they explained away its spiritual meaning. Christ showed the full meaning of this commandment; according to which we must be judged hereafter, and therefore ought to be ruled now. All rash anger is heart murder. Our “brother” here indicates any person, status notwithstanding, for we are all made of one blood. “Raca,” is a scornful word, and is evidence of pride: “Thou fool,” is a spiteful word, and denotes hatred. Malicious slanders and censures are lethal poison. (Freeman, J. M., & Chadwick, H. J. Manners & Customs of the Bible. 1996. Whitaker House)

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History of a Murder - The beginning of strife is like releasing water; therefore stop contention before a quarrel starts. –Proverbs 17:14

The newspaper reported a tragic incident of violence that took place in a South American country. A peasant killed his best friend while they were arguing about political differences. When asked why he did it, he replied with these chilling words: "We began peacefully, and then we argued. I killed him when I ran out of words."

This tragedy calls to mind Jesus' teaching in Matthew 5 about the close connection between anger and murder. First He stressed the seriousness of anger (Mt 5:21, 22). He warned that malicious anger, like murder, would be punished by God. Then He gave practical advice designed to defuse anger in a person who has something against another (Mt 5:23, 24, 25, 26).

We should all take a close look at our inner rage. We might think we can control it. Unfortunately, though, our feelings of hostility often control us and cause us to do things we would never do while thinking clearly. That's why unresolved anger needs to be seen as a time-bomb that can explode, destroying ourselves and doing irreparable damage to others. True, not all anger is wrong. But all wrong anger needs to be acknowledged and confessed before it leads to "murder." –M R De Haan II  (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

When I have lost my temper,
I've lost my reason too;
I'm never proud of anything
Which angrily I do. –Jackson

He who conquers his anger conquers a strong enemy.

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J C Ryle...