I suppose we don’t really know without being confronted with the situation, but select based on your gut feeling if you could possibly conceive of doing it.
I used to think I could without even flinching, if I had to. Now I have more doubt. But if I had to, I would do it.
Ticked all the Yesses.
If the OP had specified only slow methods of killing, then it would be a different story, but I know I can be sufficiently crazy for the split second required to pull a trigger.
I doubt I could live with myself afterwards though, even if it was self defence.
Yes.
I, as Benvolio, killed my friend, playing Tybalt, in a middle school production of Romeo and Juliet. So I know I’m up to it.
Who do you want killed? 
As I responded to the security officer at US Embassy Cairo when he asked me if could shoot somebody trying to breech the walls: Only if he’s within 400 yards. Outside of that, the shot gets tricky.
Why, what have you heard?
I suppose if someone was standing right over me threatening my life, and I had a gun in my hand, then yes, I could kill in self-defense. Any other scenario it’s doubtful. I’m just not a killer.
Yep. Made my peace with that decision before I ever bought a firearm.
you left out for money
I’ve never been in any situation where it’s been even a remote possibility, but I am certain that if it was necessary I could take a life.
That would fall under “Yes, in some other situation not listed”. I didn’t want it to get too psycho.
Absolutely.
You left out the option for “Yes, I already have”. Not that that’s relevant to me of course…
Well, I’d try. The chances of my being successful at it are somewhere between Slim and None – and Slim just left town.
I’m fairly sure I could if I felt it was necessary and ethically acceptable. The idea doesn’t cause me any particular discomfort at least. But since such a situation has never came up I can’t be sure, of course.
Yes. The most likely scenerio would be if I confronted an intruder in my home while I was armed, and he said “you’re not going to shoot me”, and then advanced on me.
What? No love for the Ladies?
Same here. Hope I never have to do it, but I can.
Yes. Been in a couple of situations where I actually tried. Never completed the act due to interference by others.
Both times it was to protect someone I cared about.
Yes. To protect my life or a loved one’s life, absolutely and quite probably without hesitation. I value life highly, but I value mine and my loved ones’ over any potential assailant’s. I count my cats among my loved ones–though I would obviously be much less likely to kill to protect pets than human loved ones. If you do something particularly cruel to one of them and dare me to respond, though, all bets are off. At that point I’d probably go temporarily psycho and end up in jail.
As for war–I’d have a much easier time killing an invader threatening the US than I would killing someone in a war on foreign soil–and an even harder time killing one in a war I didn’t really believe in. Probably a good thing I’m not in the military.