The biggest genocides were committed by these atheist regimes -

http://en.wikipedia.org/wiki/Mao_Zedong

http://en.wikipedia.org/wiki/Joseph_Stalin

Here's one excerpt, "Raised in the Georgian Orthodox faith, Stalin became an atheist. He followed the position that religion was an opiate that needed to be removed in order to construct the ideal communist society. His government promoted atheism through special atheistic education in schools, anti-religious propaganda, the anti-religious work of public institutions (Society of the Godless), discriminatory laws, and a terror campaign against religious believers. By the late 1930s, it had become dangerous to be publicly associated with religion.[97]"