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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Actually khans empire was pretty chill after everyone just said "k, khan, we give up" it was said that a man could walk from one side of the empire to the other with a golden plate on his head and not worry of being robbed. There's a reason why the time period after khan murdered just about everyone is called "pax Mongolia" or Mongolian peace.