Yeah, but were these oppressive empires really engaging in largescale genocide before WWII or was it (partially) caused by WWII? If I remember correctly, before WWII the "final solution" was supposed to be the Madagascar Plan, not The Holocaust.

The Madagascar Plan was never seriously considered, and when it was considered the war had already started (they were discussing it in 1940). People were already being placed in concentration camps in 1939. Moreover, the Madagascar Plan was a plan specifically for dealing with the Jews. The gays, Roma and other groups were not covered under that. Whether the Holocaust of the Jews was what the Nazi high command intended from the beginning is a matter of some debate. But given their general attitude even if Britain had quickly fallen, I have trouble believing that the Nazis would have stopped with putting all the European Jews on Madagascar, aside from the terrible loss of life that would have occurred in forcively adding millions of people to an environment with minimal infrastructure to support them.

Your point about the Soviet Union seems to be a valid one.

There is a more general problem here: There are some occasions where defensive wars need to be fought simply because if one doesn't then the enemy will wind up at your doorstep. World War 2 seems to have been such a war for much of Europe. That's less so for the US. But the overall point that having the war was better than the alternatives seems clear.