For Heaven's sake, the persecution of Christians by the USSR is common knowledge. It was not just about political opponents or Kulaks, many were also killed for spiritual reasons.
They were not the only one, actually and you don't necessary have to be a Communist to be kill in the name of atheism. In 1792 the French Republicans massacred the Vendeans because they just wanted to live their faith. They were atheists and liberals.
Did Stalin kill all those Christian because he was an atheist, or because he was an anti-religious Communist nut?
Is there a logical path that leads from "I don't believe there is a God" to "We should kill all those priests"? There isn't. However, one needs some admirable mental gymnastics not to consider there is a pretty clear path that leads from "I am a Christian" to "homosexuality is an abomination", for instance.
I find it striking though that you feel the need to dig up Cold War dictatorships or late 18th century revolutionary movements to prove your point that atheism is as vile as it gets.
How about here and now? How about the direct correlation between secularism and human development?
In any case, this all comes from the religion-based view of atheism as an ideology. Religion (or communism, since we're at it) are ideologies. Atheism is nothing but the lack of theism. Just as a-astrology, a-dragonism or a-pastafarinism aren't ideologies, atheism isn't one either. There are atheists who are good people and atheists who are bad people, for the simple reason that there are good people and bad people.