but don't forget, Russia was and remains overwhelmingly Eastern Orthodox.
I think you are deluding yourself. Russia during Soviet time weren't eater Orthodox or anything in general, revival of Christianity stated after Soviet regime crumbled.
As regards sheer numbers of slaughtered, again, Hitler murdered out of purely ideological-biological reasons. All full Jews had to be systematically eliminated; Stalin murdered only those whom he saw as a threat. the result may have been the same,
Hitler murdered those he saw as a thread as well.
Difference between them is simple Hitler was getting more and more insane and sick, prone to emotional reactions and decisions, probably didn't fully comprehend consequences of his orders.
Whereas Stalin was a cool, very intelligent and composed,but he was a psychopath.