adolph hitler Depends on how you mean that; by order, by assisting or by personally 'pulling the trigger.' The answer to the latter is Thomas Ferebee, though it is probably unfair to hang that label on the soldier following the order. Moa Ts-tung is credited with the deaths of up to 70 million of his own people. I believe the total death toll for WWII was "only" 20 million. Second place is Joseph Stalin.
In addition, the Mongol Invasions of both China and Central Asia/Middle East in the 1200s resulted in massive numbers of civilian deaths as the Mongols frequently would massacre the entire population of any city that refused to surrender to them. Total number is unknown, but estimates start in the 10s of millions.
Clearly Stalin and Mao lead the list, and worst of all the list is mainly there own citizens.