Honestly, this thread is the reason that I can kind of "understand" people like the Columbine kids or the Oslo nutjob. Clearly, considering senseless violence is perfectly normal, and it's just some mental check, or even just selfish self-preservation, that keeps people from acting on these impulses. So all it takes is some biological difference that eliminates that filter, and you've got yourself a Grade A sociopath.
With that said, my most recurring thought is when I'm sitting in an enclosed room with coworkers or students, and I start to wonder how many I could kill with just the pen in my hand until I'm subdued. I don't ever actually act on it because, well, I'm not a mass murderer.