Why did the Old Testament command to kill?

The old testament and the new testament take radically different approaches to righteousness. The Old Testament set up a Law which must be followed to live a perfect life and be saved. It set an impossible standard which could not be met. It also sought to isolate Israel from corruption by removing those who would have influences that would take them away from God. In fact, the passage in Deuteronomy 18 indicates as much

9 When you enter the land the Lord your God is giving you, do not learn to imitate the detestable ways of the nations there. 10 Let no one be found among you who sacrifices their son or daughter in the fire, who practices divination or sorcery, interprets omens, engages in witchcraft, 11 or casts spells, or who is a medium or spiritist or who consults the dead. 12 Anyone who does these things is detestable to the Lord; because of these same detestable practices the Lord your God will drive out those nations before you. 13 You must be blameless before the Lord your God.

This was however an impossible standard as seen by the repeated failing of Israel to live up to the Law. While I don't have scripture that directly supports it, the general purpose from a general reading of scripture seems to be that Israel was an object lesson in the state of Man. Even set-apart for God, blessed heavily for following and isolated from external negative influences, they still repeatedly failed to follow after God.

The New Testament then takes a radically different tact. Having proved that Man can not save themselves or attain their own righteousness, God instead enters in to the world through Jesus and provides salvation for us. No longer is the strict isolation and extreme consequences necessary because it is no longer necessary to provide that level of isolation from wrong doing. Christ entered in to the world and walked among it and provided his perfection. He didn't need to be isolated from sinners to try to be righteous.

Since there is no longer a need to isolate God's chosen people, there is no longer a need to kill those who would act as an external corrupting influence since Christ is incorruptible.

There seems to be a great misunderstanding here. There is no difference in the punishment between the Old and New Testaments. The differences are only in the immediacy of imposing punishment, and who makes that judgment.

AS far as the immediacy goes; we are given the parable of the tares, by Jesus himself to illustrate that judgment is delayed, because the situation has changed and after the crucifixion there will be a new covenant whereby man may become sinless in the sight of God through acceptance of Salvation;

Mat 13:24 5through 30 Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field: But while men slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also. So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares? He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them. Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.

In this parable Jesus is telling us that selection to enter God's Heaven;

Is going to be like the farmer sewing good seed. Or in other words, God created man
in his own image and that was unspoiled by pride and false desire.

But Satan sneaked into his creation and spoiled that by enticing man to Disobey God.

So as the World progressed both good and evil flourished.

And when the Angels saw the depravity of the world the asked God if the should go
throughout the Earth dispose of the Evil people.

God's answer takes a bit of understanding the nature of God and his omniscience.

God knew exactly what the future held and because of that pre knowledge, told the
Angels to just allow both Evil men and good men to not be punished until the time
for harvest and at that time judgment would be made concerning their righteousness.

This delay in judgment was necessary since annihilation of Evil would preclude the
opportunity for later acceptance of Salvation.

As far as the change of judges goes it appears that God in his mercy has decided that in order for the judgment of man to be truly just it would take someone who had been through the same temptations man faced.

Therefore God Sent his only begotten son into the world to live a life of temptation in order to evaluate man's response to that temptation. Thus Jesus has all of the qualifications necessary to be our judge.

Personally, I think that the primary difference is that the New Testament "gives to Caesar that which is Caesar's and to God that which is God's". There is, in essence, in the Christian faith, a separation from what is legal and what is right in faith in Christ. Whereas the Old Testament is both the law of God (those things that an individual should or should not do), which Christians still believe, and the law of the land of Israel (the legal consequences for disobeying the moral law). Christians no longer adhere to the latter because they are dispersed among the nations (we don't stone people anymore for violating the moral law, we obey the legal system that is over us). The same would technically be true of Jews. Jews are not stoning adulterers in the United States but allowing the United States' legal system handle those things. Or if we prefer Biblical cases: Daniel did not stone the Babylonians for the evil they did when he was in Babylon. Daniel kept to the moral law for himself...the law is good...but he did not pursue Israeli civil and criminal punishments against the Babylonians.

When Jesus refused to stone the woman caught in adultery, He did not say that the law against adultery was bad, and he did not say that stoning was wrong. However, he did not stone the woman. There seem to be two grounds for this: 1) It was against Roman law for them to stone the woman, and 2) Mercy and forgiveness are greater than the law of sin and death. While it is not wrong to punish someone for their sins, it is also not wrong to forgive, and forgiveness is greater. Pretty much the entire book of Romans will do more justice to this idea than I can give here.

Two additional things to note:

• Jesus did not abolish the law (Matt 5:17).
• Jesus did not lower the bar of morality but raised the bar. Now lust is adultery (Matt 5:28), etc.

As to specifically killing. In the United States, we do kill 1st degree murderers at times, and it's legal to kill in self-defense. Many Christians support these laws and would kill in self-defense. Though, some Christians like the Amish would not even kill in self-defense to my knowledge.

Ultimately, regardless of the laws of the land, the Christian is charged to do what is right, which is never illegal in any country:

"But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law." (Gal 5:22-23, emphasis added)