[InclinedPlane]

Psychopaths kill plenty of people though. But it helps that the vast majority of people are not psychopaths, and they react when someone does something like kill someone. If you murder someone in front of someone else your opportunities for surprise go out the window, and so much of our lives are lived in public (long stretches of private time notwithstanding). The other thing is that humans, even psychopaths, are sentient. They have feelings, they need a motivation, a reward for killing. And suicidally trying to kill as many people as possible is, in most cases, not in the cards, which is another limiting factor. If you took, say, 1 ppm of all humans on Earth and suddenly exchanged their brains for typical movie "monster rules" then things would get very ugly very fast, but not indefinitely, they would be contained, killed, brought under control by the majority of sane folks.

Nevertheless, look at folks like Ted Bundy who had a very specific target for his murders and yet still killed dozens of people in his life.