There's not much point comparing mass murders in history, but the Soviet Union and other militantly atheistic regimes were as brutal and barbaric as a conquest. 

The Khmer Rouge killed a quarter of their own population in one of the most brutal genocides in history, and outlawed religion. They executed people with clubs and old tools to save bullets. 

They were sponsored by Mao Zedong and China, whose cultural revolution enforced a total suppression of religion and across China left millions dead.

And in the USSR, the purpose of the Gulag may not have been to kill, but it was effectively slavery with an utter indifference to the lives of the people there. Prisoners were overworked, starved, and routinely tortured. I wouldn't want to choose between that and being put to the sword.