Adolf Hitler could be said to be responsible for World War II, in which around 60 million people were killed, including up to 25 million military dead and 6 million Jews systematically murdered on Hitler's orders.
Joseph Stalin ordered the execution of close to a million people, and the deportation of many more to the Gulags where around 2 million died. Some historians estimate much higher figures. Some also blame Stalin for the 1932-33 famine, which killed 6-8 million people.
Mao Zedong led China through a series of political and social programs, including the Great Leap Forward and the Cultural Revolution, which caused the deaths of as many as 70 million people through famine and starvation.