Tie them up and make them watch BB with me. Force them to call long-distance to get those votes in! Probably give them **** rations too.
More seriously, we're civilized mammals with the ability to discern right from wrong. Killing isn't civilized, it's more in the realm of animals, and wartime. We might fight or cower, depending on the circumstance. I'd like to think I could defend my family if events went that far, and I'd like to think I could spare someone if it wasn't wholly necessary.
I don't believe anyone can answer this question honestly until they've been placed in a horrible situation. And let's hope we never find that out.