God condemns the sin of the heart

Numbers 35:16-24 (ESV)
"But if he struck him down with an iron object, so that he died, he is a murderer. The murderer shall be put to death. [17] And if he struck him down with a stone tool that could cause death, and he died, he is a murderer. The murderer shall be put to death. [18] Or if he struck him down with a wooden tool that could cause death, and he died, he is a murderer. The murderer shall be put to death. [19] The avenger of blood shall himself put the murderer to death; when he meets him, he shall put him to death. [20] And if he pushed him out of hatred or hurled something at him, lying in wait, so that he died, [21] or in enmity struck him down with his hand, so that he died, then he who struck the blow shall be put to death. He is a murderer. The avenger of blood shall put the murderer to death when he meets him. [22] "But if he pushed him suddenly without enmity, or hurled anything on him without lying in wait [23] or used a stone that could cause death, and without seeing him dropped it on him, so that he died, though he was not his enemy and did not seek his harm, [24] then the congregation shall judge between the manslayer and the avenger of blood, in accordance with these rules.

God gives here a list of rules how to discern between murder and manslaughter. From verse 16 to 18 are the weapons definitely used for murder: an iron object, a stone tool that could cause death or a wooden tool that could cause death. What God is saying is that a man cannot claim to be a manslayer if he had killed with such a weapon. A man does not happen to carry such tools and kill a person by 'accident'. The weapon used proves the person's murderous intent. If the weapon was shaped in a way that could kill, no matter how much the man claims innocence of the heart, he is telling a lie. The congregation does not even need to know if the person had hated the victim, for the weapon shows it all. Likewise, outward action of sin clearly shows the sin of the man's heart. No man who is committing great outward sins can claim inward innocence. For example, no married man caught in the scene of adultery can say "But I didn't mean in in my heart" or "I still love my wife in my heart". It makes no sense. Conscious choices have been made, and the outward act clearly shows what was in the heart of the person. The sinner may convince himself and tell others that his heart is pure, but the outward action damns him. What we do shows who we are on the inside. You can judge a book by the cover, if the cover is obvious. The man who used these weapons, no matter how much he denies of the intent of the heart, is a murderer. He deserves death. Likewise it is with the sinner.

The avenger of blood shall himself put the murderer to death. This avenger of blood would most likely be a family member of the victim. This was the justice system of the time, God permitting this method of capital punishment. But in the New Testament, of course, we are instructed that we must not vengeance into our own hands, but let the wrath of God handle our enemies:
Romans 12:19-20 (ESV)
Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, "Vengeance is mine, I will repay, says the Lord." [20] To the contrary, "if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head."
God instead in the New Testament placed the government as the legal avenger to execute God's wrath upon the evil doer.
Romans 13:3-4 (ESV)
For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, [4] for he is God's servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer.
Therefore we are not to leave vengeance to our own hands anymore. This is mainly because we are recipients of God's grace. God ought to have poured out His vengeance upon us for our sins, but He has given us instead His everlasting life. How can then we take revenge? Our portion is to forgive the enemy, just as God has forgiven us. We are no longer in the place to avenge. But God will avenge, either in this world or on Judgment Day when all mankind shall be recompensed for all evil they have ever done.

From verse 20 to 21 we have less obvious weapons for murder: pushing, an object thrown, the hand. These need further examination to see if there was hateful intent in the heart or not. It is tragic here to see that even our bodies may become weapons when used with murder in our hearts. Our hands and feet were made to be used for God, to work with, and to give glory to God with. But man has used them to kill, to hurt and to destroy one another. This is the wrong design to which God has made our hands and feet. They were not made for violence, but for healing, just as Christ His feet to go the lost, and laid his hands on the sick. We must likewise offer our bodies to God that it may be used for His service, in doing good, as living sacrifices. Just as Christ's holy body was offered for our good, let us use our bodies for God's glory.

From verses 22 to 24 God explains that it is possible to push someone without enmity to death, or to hurl something at someone by accident that he dies. For example, a rock may be thrown without seeing where it would land, and kill someone. These things can happen in this fallen world. We see here that God does not condemn the outward action of the killing, because what He condemns is the murder of the heart, which is carried out in action. What we can learn from this is then that outward action does not always reflect the inward thoughts and intents of the heart. If it is not obvious, we cannot always judge by outer actions of a man, though they may appear sinful. We may judge wrong, for we do not know what is really inside a man. God knows all things that is inside a man, and only He shall judge righteously, and reveal the secrets of the heart.

Sin of the Christian can be likened to manslaughter. A true Christian who has been born again never sins with a full heart. Like the manslayer who kills a man, yet did not have enmity against him, Christian sins against God, but does not have enmity against God in his heart. The Christian loves God's laws, His will and His promises, because he is a new creature in Christ, and he has been given a new heart with new desires. Yet the Christian sins, doing that very thing he hates. By the outward actions he is sometimes opposed to God, but his heart delights in God always. As perplexing as it may seem, this is the Christian dilemma:
Romans 7:15-20 (ESV)
For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. [16] Now if I do what I do not want, I agree with the law, that it is good. [17] So now it is no longer I who do it, but sin that dwells within me. [18] For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out. [19] For I do not do the good I want, but the evil I do not want is what I keep on doing. [20] Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me.

There is no condemnation to the Christian, because though he sins, he sins not with his heart. But that is not to say that Christians are not chastised for their sins, just as manslayers were not let go as free men immediately, but had to stay confined in the cities of refuge until the High Priest died. There are consequences to sin, but not all sin leads to eternal damnation. Let us believers be thankful that God has written His laws in His heart and mind, and let us not do those things which God and we ourselves hate. Let us do good, for we love and desire good, asking God to give us the ability to do good.