Who killed more people Mao or Stalin?

Stalin is responsible for the death of 17 million Russians.

Hitler is responsible for the death of 12 million people in both concentration camps and civilians.

Alternate opinion:

Hitler: Al least 15 million Russian civilians die during Hitler's invasion in USSR. Lots of people in other countries too.

Stalin: about 300000 (rounded up) dead during party purges, about 200000 captured Nazi soldiers (died 1945-53).