I'm a pretty strict pacifist. There is absolutely no way I could ever kill anyone, regardless of the situation. If someone has a gun to my head, then I'll be like "Bye bye, world!" and hope for the best >_>
Seriously though, as antireconciler said, you can practice self-defence without ever killing someone. I'm not that keen on physically attacking someone either, but I would certainly try non-lethal ways to protect others if I could, but killing is never the answer in my mind. To me, there is not a single thing in the world that could justify it, as there is always an alternative. I can't put a value on life, even those that would do loved ones harm. That's just me
It's probably not a logical belief, but its one of the strongest ones I have. Everyone has reasons for why they do the things they do, every action and belief can be explained, and thus I would not want to hate or blame someone if they took an extreme action. It surprises me how many F types I have met (Not necessarily NF's) that really believe in punishing the 'evil' and then give a very loose definition of what 'evil' is. You don't solve anger, hate and death with more anger, hate and death.
But, I would never put myself in a position where I would be expected to kill someone else in the first place.