Googled it
Atheism.
1- China:
"The People's Republic of China was established October 1, 1949. Its government is officially atheist,"
http://en.wikipedia.org/wiki/Religion_in...
Women in China are treated like slaves, concubines, and prostitutes. With more than a billion people, China has too many men. According to the latest census, an average of 120 boys are born for every 100 girls, the greatest imbalance in the world.
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#2- North Korea
"North Korea is an atheist state, and Government policy continues to interfere with the individual's ability to choose and to manifest his or her religious belief."
http://en.wikipedia.org/wiki/Freedom_of_...
"The Government deals harshly with all opponents, including those who engage in religious practices deemed unacceptable by the regime. An estimated 150,000 to 200,000 persons were believed to be held in detention camps in remote areas, many for religious and political reasons."
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#3- Cuba
"Officially, Cuba was an atheist state from 1959 to the early 1990s"
http://www.state.gov/r/pa/ei/bgn/2886.ht...
"Cuban dissidents face arrest and imprisonment. In the 1990s, Human Rights Watch reported that Cuba's extensive prison system, one of the largest in Latin America, consists of some 40 maximum-security prisons, 30 minimum-security prisons, and over 200 work camps.[92] According to Human Rights Watch, political prisoners, along with the rest of Cuba's prison population, are confined to jails with substandard and unhealthy conditions."
http://en.wikipedia.org/wiki/Cuba
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#4- The Soviet Union
"The Soviet Union was the first state to have as an ideological objective the elimination of religion[1] and its replacement with atheism as a fundamental ideological goal of the state.[2][3] Toward that end, the communist regime confiscated church property, ridiculed religion, harassed believers, and propagated atheism in the schools"
http://en.wikipedia.org/wiki/Religion_in...
"The butchers who ran the Soviet Union killed between 25 million [The Black Book of Communism] and 60 million [Rudolph J. Rummel] innocent humans - men, women and little children. "
" Lenin and Trotsky killed 4 million people - men, women and children - by mass executions, death camps, and state-caused famine. See [The Black Book of Communism] for a good introduction to their genocide, which started as soon as they got into power in 1917. "
Obviously this assumes Atheism is a religion itself. Something that is arguable. Either way, the lack of religion has led to the most death from what I found...