In Matthew 5:21-26 Jesus amplifies the meaning of the sixth commandment "thou shall not kill." He brings out that to commit murder means more then just killing someone, it means having an angry and unforgiving attitude towards them:
"You have heard that it was said to those of old, 'You shall not murder, and whoever murders will be in danger of the judgment.' But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment. And whoever says to his brother, 'Raca!' shall be in danger of the council. But whoever says, 'You fool!' shall be in danger of hell fire.
"Therefore if you bring your gift to the altar, and there remember that your brother has something against you, leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift. Agree with your adversary quickly, while you are on the way with him, lest your adversary deliver you to the judge, the judge hand you over to the officer, and you be thrown into prison. Assuredly, I say to you, you will by no means get out of there till you have paid the last penny." (Matthew 5:21-26, NKJV)
The apostle John elaborates on this by writing that to hate someone is the same as murdering them:
"Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him." (1John 3:15, NKJV).
Murder like all sin, beginnings in the human mind (Matthew 15:18-19; Mark 7:20-23) it starts as a thought, in this case hatred, which leads to the action of murder (James 1:13-15; 4:1-3). The opposite of hating someone is loving them, we should even love our enemies (Matthew 5:43-48), seeking not revenge, but looking for ways to help them (Romans 12:17-21).
As we have seen, under the Old Covenant God allowed humans to kill other humans under certain circumstances. But what about today, and those who are now under the New Covenant, should we participate in the execution of a murder or an adulterer? Should Christians involve themselves in warfare? I would argue that Christians should not participate in such activities because the New Covenant is a covenant of life, not of death (2 Corinthians 3:4-11). Under the New Covenant Christians do not execute people for sinning. The most drastic steps anyone can take against an unrepentant Christian are withholding brotherly fellowship from him until he repents (1 Corinthians 5:9-13; 2 Thessalonians 3:14-15); and delivering or handing him over to Satan for spiritual correction (1 Corinthians 5:1-5; 1 Timothy 1:18-20).
Under the New Covenant a change occurred in the meaning of 'thou shalt not kill.' Just as Christians are to no longer execute sinners, so they should not wage carnal war, but spiritual warfare (John 18:36; 2 Corinthians 10:1-6; Ephesians 6:10-18; 1 Timothy 1:18-20; 6:11-14; 2 Timothy 2:3-5; 4:6-8). Christians must be peacemakers (Matthew 5:9, Romans 14:19), forgiving those who do them harm (Ephesians 4:29-32; Colossians 3:12-14; Matthew 6:9-15; Mark 11:25-26), treating their enemies with love (Luke 6:27-36) and not seeking revenge (Romans 12:17-21; 1 Peter 3:8-12). Hatred, which is the same as murder (1 John 3:15), is unforgiving, vengeful and hostile towards one's enemies.