I did press "yes", though it would depend on the situation. I'd be more likely to kill a "baddie" than someone more or less pleasant out of so-called mercy. But I'd be much more likely to murder people than the Doctor would, for example.
You know, I was gonna say no, but I voted "Depends" instead. :L You see, right now I'm a bit young to have ferocious enemies. If it was a last resort, yes, but I hope I will never ever have to! What about you?
There is only one scenario I would bring myself to kill someone, and that is if it was to prevent further suffering. Imagine a scenario out of something like Hostel or Saw, where people are going to be brutally tortured. I'd find the nearest sharp object and slash as many peoples necks as quickly as I could before the captors could stop me.
Other than something like that though, I would never willingly kill another. I might shoot them in the ass, but not kill them.
There are a lot of things to consider in this. Does my life depend on it? Does someone else's life depend on it? What would the consequences be? How would I kill them? Would anybody be watching? etc. etc. All these things have an affect on what our choice would be, and before it actually comes to that, it's very hard to tell what we would do in that situation. So many things to consider. So many things...