Is it true that atheism has killed more people then organized religion?...

[Minimalist]

And we wouldn't get an atheist telling his congregants

Atheists do not have congregants.

con·gre·ga·tion (knggr-gshn)
n.
1. The act of assembling.
2. A body of assembled people or things; a gathering.
3.
a. A group of people gathered for religious worship.
b. The members of a specific religious group who regularly worship at a church or synagogue.

We don't have churches. Or priests. Or holy days. Or bingo. Or child-molesting parties.