Not all crimes by atheists are due to atheism.
Not all crimes by theists are due to theism either, but some clearly are because they're done in the name of religion. The Crusades are a decent example, though their scale is disputed. The British knights went to the Middle East to take Jerusalem back, not for England, not for any kings, but for Christianity. They had crosses on their tabards, on their armour and on their shields. Their straight swords were often held upside down as crucifixes. They had to pray to become knights, and they prayed before battle. This is how you know the deeds were done in the name of Christianity.
Hitler and the Nazis were officially Catholic right to the end, but their deeds weren't done in the name of Christianity. They were done for National Socialism. I accept that just fine.
Now try to think of a battle fought "in the name of atheism". What does an atheist flag look like? What war song insults a god without offering another in its place? It's a silly idea.
Think of Communism in particular. It requires atheism, but it does not logically follow from atheism or else any atheist who really thought about it would be a Communist.
When Karl Marx wrote "religion is the opiate of the people", he was commenting on its effect on the populace, not its truth or falsehood. He thought of it as a painkiller. If religion were removed, he reasoned, the people would feel their pain sharply and be spurred to revolt. Afterwards they wouldn't need religion anymore, because in the new Golden Age they'd be happy without it.
Of course that's not how it turned out. Religion persisted because the people weren't happy at all under the new regimes. This was an indication to the outside world that the system wasn't working. It was good PR to squash religion as quietly as possible.
Also, the behavioural ideology of Communism is specific enough to conflict with that of any known religion. The authorities wouldn't take that kind of argument on their own turf. Communism became incompatible with religion on strict principle.
Finally, Communism is itself a quasi-religion. Its leaders are worshipped and even prayed to, its rules are absolute and its texts are immutable.
The faithful Russian, Chinese and North Korean soldiers and politicians who committed atrocities were atheists working in the name of Communism, just as the Nazis were Christians working for Nazism. Neither belief nor non-belief is accountable for either.
I think the problem is that people think because any country with a state religion or equivalent invokes that religion when going to war to boost approval, non-religious countries do the same thing with atheism. Atheism doesn't even work like that; it would be a terrible motivator all by itself.
- SmartLX