The arrest of Pinochet in 2000 brings up the issue of which other leaders should be or should have been tried for atrocities committed during their rule. Here is a tentative list of modern dictators (and assorted mass murderers) and the estimated number of people killed by their orders (excluding armies they were formally at war with). In Stalin's and Mao's cases, one has to decide how to consider the millions who died indirectly because of their political decisions. The Chinese cultural revolution caused the death of 30 million people (source: the current Chinese government), but many died of hunger. Stalin is responsible for the death of 17 million Russians, but only half a million were killed by his order. Khomeini sent children to die in the war against Iraq, but it was a war, so they are not counted here. The worst genocide of recent times was committed by many hutus, not just by their leader. Needless to say, I make a big distinction between killing soldiers and killing civilians. The US killed three million people in Vietnam, but the vast majority were either regulars of north vietnam or vietcongs. I don't count those as victims of atrocity. When American presidents decided to bomb the rice fields in North Vietnam, knowing that they would only kill women and children, those are counted as genocide. (Read the end of this page for why the nuclear bombs are not considered genocide).

http://www.scaruffi.com/politics/dictat.html

The worst genocides of the 20th Century
Mao Ze-Dong (China, 1958-61 and 1966-69) 49,000,000 ("great leap forward" and "cultural revolution")
Jozef Stalin (USSR, 1934-39) 13,000,000 (the purges)
Adolf Hitler (Germany, 1939-1945) 12,000,000 (concentration camps and civilians WWII)
Hideki Tojo (Japan, 1941-44) 5,000,000 (civilians WWII)
Pol Pot (Cambodia, 1975-79) 1,700,000
Kim Il Sung (North Korea, 1948-94) 1.6 million (purges and concentration camps)
Menghistu (Ethiopia, 1975-78) 1,500,000
Ismail Enver (Turkey, 1915) 1,200,000 Armenians
Yakubu Gowon (Biafra, 1967-1970) 1,000,000
Leonid Brezhnev (Afghanistan, 1979-1982) 900,000
Jean Kambanda (Rwanda, 1994) 800,000
Suharto (East Timor, 1976-98) 600,000
Saddam Hussein (Iran 1980-1990 and Kurdistan 1987-88) 600,000
Yahya Khan (Pakistan, 1971) vs Bangladesh 500,000
Fumimaro Konoe (Japan, 1937-39) 500,000? (Chinese civilians)
Savimbi (Angola, 1975-2002) 400,000
Mullah Omar - Taliban (Afghanistan, 1986-2001) 400,000
Idi Amin (Uganda, 1969-1979) 300,000
Benito Mussolini (Ethiopia, 1936; Yugoslavia, WWII) 300,000
Mobutu Sese Seko (Zaire, 1965-97) ?
Charles Taylor (Liberia, 1989-1996) 220,000
Foday Sankoh (Sierra Leone, 1991-2000) 200,000
Slobodan Milosevic (Yugoslavia, 1992-96) 180,000
Michel Micombero (Burundi, 1972) 150,000
Hassan Turabi (Sudan, 1989-1999) 100,000
Jean-Bedel Bokassa (Centrafrica, 1966-79) ?
Richard Nixon (Vietnam, 1969-1974) 70,000 (vietnamese civilians)
Efrain Rios Montt (Guatemala, 1982-83) 70,000
Papa Doc Duvalier (Haiti, 1957-71) 60,000
Hissene Habre (Chad, 1982-1990) 40,000
Chiang Kai-shek (Taiwan, 1947) 30,000 (popular uprising)
Vladimir Ilich Lenin (USSR, 1917-20) 30,000 (dissidents executed)
Francisco Franco (Spain) 30,000 (dissidents executed after the civil war)
Lyndon Johnson (Vietnam, 1963-1968) 30,000
Hafez Al-Assad (Syria, 1980-2000) 25,000
Khomeini (Iran, 1979-89) 20,000
Guy Mollet (France, 1956-1957) 10,000 (war in Algeria)
Paul Koroma (Sierra Leone, 1997) 6,000
Osama Bin Laden (worldwide, 1993-2001) 3,500
Augusto Pinochet (Chile, 1973) 3,000
Rafael Videla (Argentina, 1976-83) 3,000
Al Zarqawi (Iraq, 2004-06) 2,000