It’s extremely difficult to get them out once they get their foot in the door…

High Casualties & Initial Failures: The Red Army suffered devastating losses in the early war, partly due to Stalin's purges of officers and poor planning, yet they rebuilt and continued fighting, inflicting the majority of German casualties.
Stalin's Brutality: The regime itself used terror and enforced extreme discipline, with severe punishments for soldiers, blurring lines between fighting for survival and political objectives.
Widespread Atrocities: As the Red Army pushed west, soldiers committed widespread crimes, including rape, looting, and summary executions, often as retribution for German atrocities, but also as systematic violence.
Ideology vs. Reality: While fighting fascism, the Red Army also represented a communist ideology that imposed new forms of oppression in occupied territories, leading to fear and hatred among local populations.

Both Stalin and Hitler and their regimes were ultimate examples of supreme evil. I’d have to lean slightly more towards Stalin and his regime as being the worst of the two. Mainly because of what they did to their own country and its people.
Meanwhile, Mao (and his particular brand of evil) was just getting started on trying to outdo the other two.

Pol Pot oudoes Mao Zedong in evil

I always forget about him in relation to the other three. I guess it’s because he wasn’t in power as long as they were. He did a whole lot of evil in a relatively short period of time, compared to the other three demons.