Listed in order from highest kill count (40 million) to lowest kill count (1.5 million).
[i]Edit: Khan was more of a conquerer, rather than a dictator. But he still killed millions nevertheless.[/i]
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Khan wasn't really evil. Just exceptional at military arts, and held nothing back from that. He actually instituted fairly good policies throughout his empire, and gave anyone he fought a fair chance of surrendering and being absorbed peacefully before the pillaging. I voted Stalin, but I'm really amused by how few votes Mao is getting.