The Sixth Commandment

The sixth commandment says "You shall not murder." Exodus 20:13
There is a distinction made between intentional and accidental killings. It is only the intentional killing that calls for the death penalty.  Exodus 21 says that being careless with something dangerous(a bull) that causes the death of a man calls for the death penalty.

Jesus also said in Mathew 5:21-26  that being angry is cause for judgment. He says to reconcile with your brother before making an offering to God. And to settle matters quickly if you are taken to court.

1John 3:15 Says that "Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him."

Jesus also said to love your enemies. Killing someone isn't very loving.

Pray for them. And let God take care of it. He can do a better job than we can.