You cant evaluate that on the run can you? Say you would have a 100% chance to simply knock him out, then killing him is definetely wrong. :D
But honestly, that whole issue is much more complicated and also related to certain other "problems" in certain other countrys. lol. I dont wanna start an arguement.
P.S.: If someone is armed and threatning your family by my moral codex it is fine to "blow him out of his boots".