I have one avid follower who commented that atheists have caused the greatest mass murder in human history. I do not know where he got his statistics or whether he researched to support his claim, but clearly it is just an attempt to ruin atheism and to pass the evilness that the organized religions have done.
Atheists have long been the oppressed group of the society. Atheists in earlier centuries have suffered from the “authority” of these superior religions. We have been subject to the Inquisition of the Catholic Church.
As Christopher Hitchens say that the main cause of evil in this world are organized religion. Muslim go to wars disguised as jihad. Political leaders have used religion to oppress their people. Hitler, for one, is a believer of the Biblical god and has justified his authority as divinely inspired.
No one has killed for atheism. But millions have been killed for being atheist.