Since facts are clouded we'll never know what numbers are in the ballpark. The man who carried out and headed the execution of the jews in the final solution was Adolf Eichmann and when a friend asked him privately, and mentioning that he did so as a historian, eichmann said simply "6 million". I would think that you shouldn't count casualties that are directly related to war so stalin and hitler get some slack (for whatever that's worth

) but those two I think definetly top my list. Of course, I'm uninformed about Mao so I don't know his numbers and where to rate Mao. King Leopold II is definetly one.
Charles V and Phillip II can count for something but they didn't intend the Indians to die, I'm sure, plus they were probably not too interested or informed about conditions in the new world. This wiped out millions upon millions of native americans.
There was a dude from Cambodia, can't remember his name, but he is supposed to have killed around 2 million.
You can accuse Churchill of being a mass murderer since he embraced civilian bombing unnecessarily and we all know how important and vital the bombing of dresden was. The only reason he didn't hang was, just like Stalin, he was one of the victors.
Truman can't compete quantitatively but ooooh when those 2 mushrooms get up in the air, then it's gettin hot and ruthless. That man should hang by today's standards for using MWD that way but back then international and humanitarian moralism wasn't as present as it is today.