Which one do you believe in, and why?
Personally, I believe that its people who kill people. A gun doesn't just get up and say "Hm, I really hate that guy, I think I'm going to shoot him!" No, something has to be there to pull the trigger at a target.
I say "at a target" because guns CAN accidentally discharge, but they don't accidentally discharge at your enemies (or not normally, any ways).