Some information from this blog post.

Perpetrator Where / When Number Killed Who/How/Notes
Mao Ze-Dong China, 1958-61 and 1966-69
Tibet 1949-50
49-78,000,000
Jozef Stalin USSR, 1932-39 23,000,000 Purges plus Ukraine’s famine.
Adolf Hitler Germany, 1939-1945 (WWII) 12,000,000 Concentration camps.
Leopold II of Belgium Congo, 1886-1908 8,000,000
Hideki Tojo Japan, 1941-44 5,000,000 Civilians in WWII.
Ismail Enver Turkey, 1915-22 1,200,000 Armenians
350,000 Greek Pontians
480,000 Anatolian Greeks
500,000 Assyrians
Pol Pot Cambodia, 1975-79 1,700,000
Kim Il Sung North Korea, 1948-94 1,600,000 Purges and concentration camps.
Menghistu Ethiopia, 1975-78 1,500,000
Yakubu Gowon Biafra, 1967-1970 1,000,000
Leonid Brezhnev Afghanistan, 1979-1982 900,000
Jean Kambanda Rwanda, 1994 800,000
Saddam Hussein Iran, 1980-1990
Kurdistan, 1987-1988
600,000
Tito Yugoslavia, 1945-1987 570,000
Sukarno Communists 1965-66 500,000
Fumimaro Konoe Japan, 1937-39 500,000? Chinese civilians.
Jonas Savimbi Angola, 1975-2002 400,000
Mullah Omar – Taliban Afghanistan, 1986-2001 400,000
Idi Amin Uganda, 1969-1979 300,000
Yahya Khan Pakistan, 1970-71 300,000 Bangladesh
Benito Mussolini Ethiopia, 1936
Libya, 1934-45
Yugoslavia, WWII
300,000
Mobutu Sese Seko Zaire, 1965-97 ?
Charles Taylor Liberia, 1989-1996 220,000