Do atheists have the moral grounds to make these accusations against religion?

Athiests often attack theists by saying that religion has been the cause of violence. However, the two most murderous political regimes in history have been explicitly athiestic ones (Lenin & Stalin's Russia, Mao's China). The third most murderous was a Darwinist one (Nazi Germany, which was based on the belief that the Aryan people were the most evolved and that all lesser evolved people should be eradicated), and it started the most widespread, destructive, and deadly war in history. And, when it comes down to it, there aren't many more explicitly athiestic or darwinist political regimes that have existed. Since athiesm and darwinism have been the causes of the greatest period of violence in human history, can athiests use past episodes of theistic violence to argue against religion?