Whilst I’m not averse to the notion that in a war where one bunch of murdering genocidal thugs who called themselves National Socialists slaughtered, and were slaughtered by, another bunch of murdering genocidal thugs who called themselves International Socialists, Americans can feel the right to say in the words of Kissinger about the Iran/Iraq war “pity they both can’t lose” but that was not all the Second World War was about.
First off your theory that the Nazis didn’t murder their own citizens is way off, quite apart from the Jews (and even if it was only the Jews why would that not matter?), disabled people, homosexuals, devout Christians, gypsies, and pretty much any German who opposed them were systematically murdered by Hitler, and of course he also went on to murder millions of people who were not Germans and I do mean murder, not kill in the course of war time activities.
Secondly Some ethnic Germans may have suffered abuse in Poland, in the same way that ethnic minorities in every country in the world get abused from time to time but there was no systematic murder campaign against Polish Germans, this was Nazi propaganda in the same way they alleged the Sudeten Germans were suffering abuse in Czechoslovakia.
So quite possibly if the Second World War had remained a power struggle in Eastern and Central Europe between two totalitarian ideologies then indeed the Americans would never have gotten involved. However it wasn’t just that was it? Hitler invaded Belgium, Holland, Norway, Denmark, Greece, Yugoslavia and of course France. Britain and her Empire were fighting the Germans, maybe you feel the Brits should have kept out of it, strategically that may be a fair point but morally a bit difficult and anyway whether they should have or shouldn’t is a moot point, they were in.
It was very much not in the interests of America to see all of Europe under the control of the Nazis (as it was later to be not in American interest to see all of Europe under the control of the Communists), and to see the British whose Navy guaranteed the freedom of the seas fall to the Nazis would have been a global disaster for America.
Hitler declared war on the US so the issue was decided, the enemy of America’s enemy was Stalin’s Russia, it didn’t do any harm to let the two of them slog it out for a few years while the western allies prepared the liberation of western Europe, they knew they’d have to liberate the rest later but for all its dreadful faults post war Communist Eastern Europe was genuinely not as bad a place as a Nazi dominated Europe would have been.
I am not at all defending the Nazis, but if you change the name Hitler to Stalin and change the name of the countries (France, Poland etc. to Latvia, Estonia, Georgia, etc.), you get the same scenario, but with much less global geopolitical impact.
I am only making the case that Hitler, as unbalanced and deranged as he was, may well have been less of a monster than Stalin.
Let me put it this way, if you were a dentist and your assignment was, using 1945 dental technology. to help a patient with severe dental problems, including excruciating pain, would you rather work on Hitler's mouth or Stalin's mouth? To me it would have been a clear cut choice.