I mean, if we're going by body count alone then religion easily outpaces communism and Nazism.

I doubt it. Even if you sum up all Jihads, Crusades, some honor killing bullshit then it would probably still be less than both commies and nazis. Earth not so long ago was a pretty empty place. I guess more people died from plagues than religion itself.

We're not comparing the sum total of all possible means by which humans can die, we're comparing ideologies committing willful murder, and more abstractly, theism versus atheism.

Religion has been killing people for thousands of years, whereas Nazism and Communism haven't even existed for centuries.

There is no rational calculus by which one can justify banning Nazi and Communist ideology because of their death tolls and extremism and not also ban the practice of every major religion.

This trope is a great timesaver that immediately identifies someone as confidently ignorant and not worth engaging.

The "what killed most people" in general is meaningless as you can manipulate it in a way to support any statement.

And yet, there is some truth in ascribing a lot of violent deaths to communist circumstances, at least in the last two centuries. The fact that, as opposed to Nazism, in Communism killing others was never a direct aim, in practice human life somehow lost much of its value and people were dying in various ways, both individually in prisons etc., as well as en masse like in Holodomor.

Can we at least agree that Communism and Nazism are both deeply flawed ideologies that result in mass suffering, death, and destruction?

Arguing which is worse is like arguing whether you’d rather have chlamydia or syphilis.

Also for the record, censorship is not the answer to fighting against destructive ideologies. Quite the opposite, education, is the answer.