Again, the difference is that there is no justification inside the "lack of a belief in a god" for doing anything. The fact that the Khmer Rouge was horribly misguided about how to run a society did not have its roots inside of atheism. The fact that ISIS is horribly misguided about how to run a society does have its roots inside of a religious dogma because the playbook they're following is Islam. They aren't getting Islam wrong, they're getting it right, and it's the source for their entire approach to life.

Okay, here's where you keep getting mixed up. I am NOT saying Atheism causes people to do horrible things just as much as religion. This seems to be the angle you're taking. "Atheists who do bad things weren't doing them because they were atheists!" is your argument. But this betrays a fundamental misunderstanding of what I am saying.

I am saying that people are doing these terrible things because of messed up ideologies, not because of religion or non-religion. People try to create their ideal worlds- sometimes religion figures in to it and sometimes it doesn't- but it's the fanatical ideology that leads to genocide, regardless of religion or lack thereof.

All you are arguing is that atheism doesn't cause wars in a vacuum. I'd agree, but religion doesn't either. Almost all major religions preach nonviolence. Genocide is caused by twisted versions of religions or Darwinism or socialism that people bend to their political agenda or wants.

The "they're following Islam right" claim is just plain untrue. The Quran insists on violence only in self defense under oppression. These people simply have decided that anyone who doesn't let the have their ideology- an islamic state intolerant to others- is oppressing them, and thus use that as an excuse to bend their religion in to a war.

It's just as much of a twist as bending Darwinism in to genocid/ethnic cleansing,, bending Marxism in to the Killing Fields to eliminate capitalist ideas, bending Christianity ("those who live by the sword willdie by the sword") in to the Crusades or Christian Dominionism, etc.

Hitler was a Catholic who believed in the occult.

Hitler elieved in a wacky, twisted version of Darwinism in which Aryans were evolutionarily superior. He was only officially Catholic by default. Regardless, that was just a name in the list and I don't want this to degrade to a Godwin's Law argument on Nazi Germany.