War with the Nazis was absolutely inevitable, Don, quite regardless of the peculiar atrocities that administration indulged in (and the actual death camps where people were being deliberately killed weren't yet running in '39...I think they got started in 1942 after the decision was made by the top Nazis to invoke a "final solution"). In any case, an emergent empire as aggressive and downright megalomaniacal as Hitler's always ends up at war with just about everyone else around it.
Napoleon was another case of that sort of thing, and it all comes down in the end to imperial checks and balances between great nations. If any one nation pushes too far and occupies too many other people's land, and becomes "too dangerous" to the old status quo, the other nations will presently join forces against it.
I fully empathize with your concerns over the Nazi atrocities and quite agree with those concerns, but that was not what the British, French, Russians, and Americans (the governments) were fighting over when they went to war (though it did serve as a very good motivator for the Allied side to enlist later in terms of believing in their cause). The primary thing they were fighting over when they went to war in '39 (British and French) or '41 (USA) was the usual imperial checks and balances, because the Germans were being way too expansionist. That always ends up triggering a war between major powers eventually. Knowledge about any Nazi death camps came much later, long after the decisions to go to war had been made. When such knowledge did come, people were very shocked by it...understandably...but it did not play a part in the initial British, French, or American decisions to go to war. As for the Russians, who had committed equally terrible atrocities against their own population in the 20s and 30s...why did we not then, by your reasoning, attack them? Why did the West not come to the aid of millions of slaughtered and innocent Russians, victims of the Stalin regime? We should have, if I follow your reasoning. Stalin killed even more civilians (his own) than Hitler did.
I'll tell you why we didn't attack Russia to save those poor people that Stalin was killing in his concentration camps. It didn't directly affect the West in terms of the overall balance of imperial power, that's why. The West didn't particularly give a damn what the Russian government was doing to Russian citizens, but they DID care about the balance of imperial power in the world. Hitler was changing the balance of imperial power in a huge way, and that's why the West fought Hitler.
It's also why the Cold War immediately followed WWII...because Russia had expanded into central Europe from 1944 on and was now a direct threat to the West in terms of the overall balance of imperial power in the world. The Cold War was an inevitable response to that, and it was fought in little proxy wars mostly, because no one was ready to face the terrible cost of an all-out war between the great powers. They went for small wars instead...and massive military spending and research.
These wars are not fought over some kind of moral issue, Don. They're fought over raw imperial power, the balance of power, and material considerations valued by imperial powers. The morality issues are raised to get the public emotionally onside to back the war and face the difficulties. Don't think the Nazis didn't have a long list of their own "morality" issues to get their people wired for war. (most of them totally bogus) That's how you get ordinary people to fight...over supposed moral issues. But if you told them it was really being done for totally pragmatic imperial interests, they wouldn't be nearly so enthusiastic about putting their lives on the line.
The USA may well end up triggering a major war eventually between themselves and Russia or China for much the same reason...an overly expansionist policy. Any nation that tries to dominate the whole world is likely to one day end up fighting most of the world. Napoleon had his comeuppance. The Germans had theirs. I expect the USA will have its too...eventually. It's been in its Napoleonic imperial phase ever since 1945, and that sort of hubris can only last so long.