The executive summary is that the Soviets murdered more in peacetime and the Nazis more in wartime; and both mostly murdered by deliberate starvation or by rounding up people and shooting them. The people of (e.g.) Poland or Belarus were caught in a vise: what you needed to do to survive under one would get you killed under the other. And slave labor, killing, and forced migrations didn't end when the war did.
Because the ideology was supreme (though Stalin was perfectly happy to use nationalism when it suited him), peasants didn't matter as much as industrialization, so they could starve--and should. And on the other side, Hitler claimed that international Jewery was the cause of the
failure delay of the Russian campaign, so the best way to win was a massive campaign to kill all available Jews.
I'd never realized how devastated Poland was.
If you want the details, read the book. If not, you'll probably sleep better if you don't read it.
I'm given to understand that the Soviets were pretty brutal when they incorporated the Chechens and other Muslim groups into the Soviet empire, but they aren't the subject of this study.
Tribes invading other tribes looking for loot and land and slaves is an old story, as is the plight of those caught in the seesaw. I don't know if it is the sheer scale, the methodical care, the fact that this is within living memory, or the insanity of the ideologies that makes this seem more horrible.