@Draug, Octavious,
Well, I disagree with you both. :-P (Though with Draug more, since I support Octavious's conclusion vis-a-vis Mujus's point).
Draug, you're not responsible for what the killer may or may not do later. You don't know that at all. All you know is what he's trying to do now, and by not defending yourself, you're saving him. What he does with that chance is his business.
After all, it is only the self-defense element that justifies the killing at all. If he had already killed somebody else, and served his sentence (perhaps a very short one due to an incompetent judge), I don't imagine you would advocate going and shooting him in cold blood in order to save his later victims. This example is just to highlight the fact that I don't really think you believe the model of responsibility that you're putting forward, because according to it, you'd be responsdible for his later killings if you DIDN'T go kill him in cold blood.
@Octavious, it's certainly relevant what that word in the Bible means, and it meant "kill" in the sense of "murder." Your assertions that it is extremely broad are simply false. We should approach the Bible with the faith of a child, but not necessarily with the exegetical skill of one.
There is another verse, though, that I'm surprised nobody has mentioned -- Jesus' command to turn the other cheek and resist not evil. This seems very on point.