Excellent question. I honestly don't know. I think you have to judge case by case.
If Isis was about to loot my town and rape my family, I would kill every single one of them. My logic being that I am forced to choose between the attackers getting a bullet in the head or having my innocent family raped and tortured to death.
Sometimes life forces you to choose amongst a series of evil-looking options, but you can't choose anything else, so you pick what seems the most fair.