1. 1949-1976 - In China, Mao Zedong killed 48 to 60 million, including the systematic torture and killing of 282,000 Mongolians 1965-1976, and 1.2 million Tibetans 1950-1976, and 15.7 million Chinese sent to concentration camps, and 27 million Chinese peasants due to starvation. The death toll does not include the 110 million infanticides and abortions in China since 1971.
2. 1937-45 - In Europe and Asia, World War II caused the death of 50 million, including the 200,000 Chinese civilians killed by the Japanese army in the Rape of Nanking in December 1937, and including Hitler's Holocaust that killed 6 million Jews, and ending with the 150,000 Japanese citizens killed by the American atomic bombings of Hiroshima and Nagasaki August 6 and 9, 1945.
3. 1492-1890 - In the Americas, 35-50 million Native Americans died of disease and warfare as a result of the European conquest begun by Columbus and concluding with the Indian wars in the United States.
4. 1924-1953 - In the Soviet Union, Joseph Stalin killed 20 million.
5. 1914-1918 - In Europe, World War I killed 15 million.
6. 1918-1921 - In Russia, civil war and famine that followed Lenin's Bolshevik Revolution of 1917 killed 8-9 million.
7. 1911-37 - In China, civil war and famine that followed Sun Yat-Sen's Republican Revolution of 1911 killed 4 million.
8. 1900-1908 - In the Congo, King Leopold and his Belgium forces killed 3 million.
9. 1950-1953 - In Asia, the Korean War killed 2.8 million.
10. 1971 - In Pakistan, General Yahya Khan and his Islamic West Pakistani army killed 1.5 to 3 million Bengalis during the War of Liberation that followed the creation of of an independent Bangladesh out of the former territory of East Pakistan. In Bangladesh during the 7 months in 1971 from March 26 to December 16, from 6,000 to 12,000 people were killed daily. By December, there were 10 million homeless refugees in Bangladesh. The 1981 report of the United Nations Human Rights Commission (UNHRC) concluded that the genocide committed in Bangladesh was one of the worst in history.
11. 1983-2000 - In the African Sudan, civil war killed 1.5-2 million.
12. 1998-2000 - In the African Congo, civil war killed 1.7 million.
13. 1915-16 - The Central Committee of the Young Turk Party in the Ottoman Empire killed 1.5 million Armenians. The Committee also from 1913-1922 killed 1 million Hellenes of Pontos and Asia Minor.
14. 1975-1979 - In Cambodia, Pol Pot and his Khmer Rouge killed 1-1.65 million.
15. 1979-2001 - In Afghanistan, the Russian invasion and civil war killed 1-1.5 million.
16. 1962-1992 - In African Ethiopia, civil war killed 1-1.4 million.
17. 1910-1920 - In Mexico, the civil war following Father Hidalgo's Revolution of 1910 killed 1.25 million.
18. 1980-1988 - In the Middle East, the war between Iran and Iraq killed 1 million.
19. 1967-1970 - the African Nigeria, the Biafran revolt killed 1 million.
20. 1976-1992 - In African Mozambique, civil war killed 1 million.
21. 1964-1974 - In Vietnam, Lyndon Johnson and Richard Nixon and the U.S. military killed 1 million North Vietnamese civilians in bombings.
22. 1994 - In African Rwanda, Hutu extremist rebels killed 600,000 to1 million minority Tutsis and moderate Hutus during the 100 days from April 6, when a plane carrying Rwanda's Hutu leader, President Juvenal Habyarimana, was shot down, to July 4, when the Tutsi-led Rwandan Patriotic Front (RPF) captured the capital city, Kigali.
23. 1954-1962 - In African Algeria, the colonial war with France killed 650,000.
24. 1945-1954 - In Vietnam, the First Indochina War with France killed 600,000.
25. 1965-67 - In Indonesia, the government killed 500,000 Communists
26. 1975-1994 - In African Angola, civil war killed 500,000.
27. 1900-2000 - In Brazil, deforestation killed 500,000 Amazon Indians.
28. 1975-85 - In East Timor, Suharto of Indonesia ordered the occuption the island and the Indonesian army killed 200,000 to 600,000 East Timorese people.
29. 1971-1979 - In Uganda, Idi Amin killed 300,000.
30. 1936-1939 - In Spain, civil war killed 365,000
31. 1991-2000 - In African Somalia, civil war killed 350,000
32. 1987-1988 - Saddam Hussein in Iraq killed 100,000 Kurdistans.
33. 1960-1981 - In Guatemala, right-wing death squads killed 100,000.

Sources:

• Davis, Mike. Late Victorian Holocausts: El Nino, Famines and the Making of the Third World. New York: Verso, 2001. 464 p.
• Kolata, Gina. Flu: the story of the great influenza pandemic of 1918 and the search for the virus that caused it. New York : Farrar, Straus and Giroux, 1999. 330 p.
• Murderers Row: Top 4 in history from John Myers
• Top-Ranked Atrocities and Wars of the 20th Century from Twentieth Century Atlas
• Worst U.S. Disasters