You shall not murder. (Exodus 20:13)
We should fear and love God that we may not hurt nor harm our neighbor in his body, but help and befriend him in every bodily need [in every need and danger of life and body]. (Martin Luther)
First a note about “murder”. The King James rendering of “kill” is still quite familiar, but “murder” is the actual meaning of the Hebrew word in the original (and “kill” probably conveyed that meaning accurately for 17th Century English). Self defense was permitted in the Law of Moses, as were capital punishment and war.
“I haven't murdered anyone, and don't intend to, so, commandment obeyed, right?” Not so fast! Jesus, in His famous Sermon on the Mount (specifically, Matthew 5:21-22), showed what fulfilling this commandment really entails. And once again (and not for the last time!), real obedience extends to our heart. Jesus said that even anger and hurling insults is wrong. Ouch! Now that is tough! Does that sound unrealistically sensitive? Murder doesn't arise from love or joy! While not entirely alone, anger and hatred are principal roots of murder. And even if one doesn't actually take violent action, anger, bitterness and hatred do harmful, ugly, things to the one who hates!
But, what about what Luther said, about helping and befriending our neighbor? Luther simply echoed Jesus' command, based on this command in Matthew 5:43-44, “But I say to you, love your enemies and pray for those who persecute you.” While it is so easy (and tempting!) to view commandments such as this one as caprices of a controlling busybody (and guess what commandment comes next!), God created human beings. God designed human beings and designed us to live socially. God isn't a Kosmic Killjoy! God knows better than we do what behaviors are healthy for us individually and will foster a healthy (pleasant and safe) society.