If Stalin’s sole motivation for having people killed was because they were religious, then having Trotsky, an atheist, assassinated wouldn’t make much sense, would it?

If Hitler was solely concerned with destroying the Jewish religion, then ethnic Jews who were non-practicing, atheist, or converted Christians would have been safe, but they weren’t. Nor were the captured Soviet commissars that were executed.

If Mao was killing in the name of atheism, then fellow atheists who believed the same damn things he did shouldn’t have wound up dead, but they did.

A lack of belief in god(s) just doesn’t work as a justification, and claiming it does is just as absurd as blaming Tojo’s lack of belief in a flat Earth, or Pol Pot’s lack of belief in purple leprechauns for their crimes. Simply believing in god(s) is obviously not enough to turn someone into a slavering murderer, especially since the vast majority of religions preach non-violence, it has nonetheless been explicitly used as a justification for murder throughout history, and more often than not, those murders have been condoned by the leaders of the religion’s organized hierarchy. The same can not be said for atheism, and that’s a damn big difference.