I think you answer the question well in post 20.
The world over the centuries has seen millions exterminated in the name of communism, nazism and yes, manifest destiny. Each of these motives were based on a political or social philosophy that rationalized the murder of millions to achieve a certain goal.
More likely, the common thread is the lack of lack of democratic structure in each of the mass killings mentioned. Since, communism lacks such a structure, it makes it easier for a leader to take such action and the people without any democratic power must take the brunt of the wrath or simply watch and hope they are not next.
Thus, it would be much easier for China to send people to death camps than it would be the US or a European democracy. The present day examples would be Sudan and Rwanda where hundreds of thousands have been killed. Not atheists, but mass murder.
Yes, indeed. I’ve noticed your interest in such discussions, and thought this might interest you.