H. H. Holmes

He was far and away the most evil man to ever exist. Every mass murderer, serial killers, or tyrannical leader that I've ever read about has had a reason to kill. Admittedly many of those reasons were sick (i.e. gratification), but they at least they all seemed to have a reason. I get the feeling that H. H. didn't for many of his countless murders. He just killed.

[To clarify, don't interpret my post as excusing the behavior of mass murders, serial killers, or tyrannical leaders in any way, shape, or form. They were all horrible people, and their crimes are inexcusable. I just think that they are less evil than H. H., but that's not saying much.]