If you don’t think radical atheism isn’t like radical Christianity and radical Islam, you haven’t studied the history of the various communist countries of the world.

People who are absolutely certain they are right can do some fucked up shit.

Do you mean anti-theism? If you’re talking about people that actively opposed religion. Atheism is simply a lack of belief in theism.

I’m also not sure how communism relates either, unless you’re talking about secular societies that were communist and committed atrocities, such as Soviet Russia under Stalin. In Soviet Russia the state belief was atheism, but again, in their pursuit to eliminate religion I would argue that is really anti-theism.