From a comment on a comic:

“Martin Luther explains that God and government are not constrained by the commandment not to kill, but that God has delegated his authority in punishing evildoers to the government. The prohibition of killing is forbidden to the individual in his relation to anyone else, and not to the government.”

My reply:

The Christian Just War Theory or Doctrine was produced and expanded upon by Saint Augustine, Thomas Aquinas, Hugo Grotius and others. (I never came across Martin Luther as a proponent of war.) Between them they did a good job of saying “That there God geezer, yeah, didn’t say ‘not kill’, he said ‘not murder’, right? And it can’t be murder if it’s not illegal, can it? And how can it be illegal if the King or Pope said to do the killing? So it’s not murder and God’s perfectly fine with it. Stands to reason, doesn’t it?”

Another argument is that killing non-believers who refuse to convert is doing them a favour by saving their souls. That was the excuse in Africa and other places where exploring and conquering was going on.

A third was that provided you are careful in your killing, then that’s reasonable. So kill people nicely and proportionately, and even Jesus would be OK with that. This argument is used by politicians today.

So, despite Holy Scripture from God, and prophets and the Son of God saying not to kill people, it is fine if a rich or powerful person tells some poor person to kill another person, because the rich and powerful said so. They know better than God, Jesus, Moses and that lot what God wants.

You’ll find the same pro-killing arguments in the Jewish and Moslem Just War arguments too.