Well first of all, I hope we're not just jumping to the "evil = body count" bullshit and actually thinking here.
Secondly; good and evil are entirely subjective. Hitler didn't see himself as evil, and frankly we wouldn't see him as the most evil man ever had the holocaust never happened(although he never actually committed any of the killings directly much like how Charles Manson's in prison for murder without having ever killed someone). Most people in Germany at the time didn't see him as evil either. He brought a broke and ruined country back after Germany got shafted for Austria's fuck-ups leading to the start of the Great War.
Thirdly; in a delightfully ironic twist more people have died and killed others in the name of Jesus of Nazareth, a staunch pacifist, than any other person. Don't believe me? Look up the Crusades which slaughtered people for having the audacity to own a city which had been in their territory for centuries. Or maybe the Inquisition that killed "heretics" by terrible methods.
But if put to the task and had to come up with a name? Gavrilo Princip, the man who assassinated Archduke Franz Ferdinand. Why him? Because his action of killing the Archduke sparks the Great War(which wiped an entire generation of people off the planet), which leads to Hitler being super pissed off(he was a soldier), which leads to him forming the Nazi party, which leads to WW2, which leads to the holocaust, which leads to the Cold War and all it's bull shit, which leads to the United States training middle eastern soldiers to fight off the "evil" communists, which leads to us giving weapons and training to Osama Bin Laden, which leads to him getting pissed that we left, which leads to him ordering the attacks on 9-11, which leads to our current fucked up situations including, but not limited to, us owing China many many trillions of dollars we'll never be able to repay. He shot one man, and in turn fucked over the entire world.
