I had similar thoughts on the matter. 1) Communism gives more plausible deniability that their mass murder was either legitimate law enforcement, or wasn't purposeful murder. They executed kulaks for stuff like "hoarding" or "economic sabotage": "shops are empty because reactionaries stole it". Stuff like Holodomor could be blamed on simply bad harvest (as opposed to criminally stupid economic policy). Same for Gulags - in theory, people were supposed to serve their sentence and be allowed to go home. If most starved to death, well, its nobody's fault (but imperialist-capitalist saboteurs) that there are shortages. So, Nazis cannot claim they didn't cause deaths on purpose. 2) Communism was big into salvation of humanity. Zealously and loudly parroting party line might not have guaranteed you'll never get bullet in back of the head, but it vastly improved your chances. But for Nazis, Jewish ghetto police and similar collaborators were simply last to be liquidated. That's no good. Even dogs understand, that once your enemy lies down on his back and show you your throat in humiliating gesture of submission, you don't bite him in the throat, you only do it if he's trying to pick a fight. Commies, like wild dogs, spared those who submitted quickly enough. It was very harsh, but fairly predictable. Nazi brutality was more random. So, Nazis cannot claim they they had no choice but to "kill dangerous and unrepentant criminals who are to blame for your family starving".
