By siding with the USSR, and letting them take control over eastern Europe? The USSR was just as bad as Nazi Germany.
As one example there's the Hunger Plan. Hitler's defeat stopped him from executing it, though they had started. He planned to kill 35-41 million people in eastern europe via starvation and take the land for Germany.
The USSR equivalent, Holomodor, killed 3-5 million, and was fully executed. Horrendous. Not as bad as the hunger plan.
And that was not the only Nazi mass death plan....
Maybe the Nazis were worse, but wasn't really a good vs evil war.