i'm guessing that the majority of people could kill someone with ease. look at the army for instance. you don't hear too many stories about people refusing to pull the trigger. but then again, it's one of those things that you don't really know till it comes down to it. as far as what you think about. dude, get help. or at least never work at a post office.
Only if they were threatening me or my family. Though I like to think I would try to wound them, when you or your loved ones are in danger animal instinct just takes over, so who knows.