Have worse atrocities been committed by secular governments or religious governments?

I mean this honestly. Scientifically.

It's not, at least at this moment, a rhetorical question. :-)

So far, based on recollections, it seems to me I've heard someone definitively state it was Stalin who is known to have killed the most people. And he did that for political reasons, there was no religious dogma or religion invoked.

However, I have not checked that statement.