James 4 NIV
1 What causes fights and quarrels among you? Don’t they come from your desires that battle within you? 2 You want something but don’t get it. You kill and covet, but you cannot have what you want. You quarrel and fight. You do not have, because you do not ask God. 3 When you ask, you do not receive, because you ask with wrong motives, that you may spend what you get on your pleasures.
4 You adulterous people, don’t you know that friendship with the world is hatred toward God? Anyone who chooses to be a friend of the world becomes an enemy of God. 5 Or do you think Scripture says without reason that the spirit he caused to live in us envies intensely?[a] 6 But he gives us more grace. That is why Scripture says:
“God opposes the proud
but gives grace to the humble.”**
7 Submit yourselves, then, to God. Resist the devil, and he will flee from you. 8 Come near to God and he will come near to you. Wash your hands, you sinners, and purify your hearts, you double-minded. 9 Grieve, mourn and wail. Change your laughter to mourning and your joy to gloom. 10 Humble yourselves before the Lord, and he will lift you up.
11 Brothers, do not slander one another. Anyone who speaks against his brother or judges him speaks against the law and judges it. When you judge the law, you are not keeping it, but sitting in judgment on it. 12 There is only one Lawgiver and Judge, the one who is able to save and destroy. But you—who are you to judge your neighbor?
Matthew 5:21-25 (King James Version)
21Ye have heard that it was said of them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
22But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
23Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee;
24Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
25Agree with thine adversary quickly, whiles thou art in the way with him; lest at any time the adversary deliver thee to the judge, and the judge deliver thee to the officer, and thou be cast into prison.
1 John 3:15
Anyone who hates a brother or sister is a murderer, and you know that no murderer has eternal life residing in him.
The difference is motive. Murder is out of hate, killing in war is out of duty.**