well, what about the assertion that by 1940 stalin had murdered more people and invaded almost as many countries as hitlers germany? its historical fact that the death camps were not implemented to carry out the final solution until 1942. so couldnt your argument also be used against the russians, we allied with stalin and he continued to butcher millions in eastern europe, as well as gaining nuclear power.
No definitely, I agree and that's my point. The war
was not fought on the basis of preventing genocide or defending human rights, whether in Germany or Russia. Those were secondary considerations for western powers, if considered at all. It served UK and later US interests to ally with Russia in 1941 in order to defeat Hitler, who presented the graver and more immediate threat, and Stalin was allowed to pursue his totalitarian project in Russia.
what if they had been left to fight each other, and both were significantly weakened, and then the allies had attacked with full force?
As I mentioned earlier, there would be no guarantee that both would be "significantly weakened", in fact, given that the full military resources of Germany could be brought to bear on Russia without other fronts (in Europe, North Africa and the Atlantic) to worry about, it is more than likely they would have defeated the Soviets, thus leading to the situation mentioned above. Given that possibility, it was inevitable Britain and subsequently the US would join the war, holding their noses to ally with Stalin, whom they hated (not for his purges and authoritarianism though, but for his 'communism').
In the unlikely and unpredictable event that Germany and Russia had both been "weakened" (however you might define that) I guess you might be suggesting something like "why didn't the allies invade continental Europe, smash Hitler and then invade Russia"? I can't say. In fact, who knows? That's the problems with "what ifs".
As it actually happened, there was a contingency plan by the British and Americans to invade Russia after defeating Germany - Operation Unthinkable - but it was deemed militarily unfeasible. Russia was too strong, having managed to turn back the Germans, helped by the UK and American bombing of Germany, allied naval blockades of supplies to Germany, the opening of a secondary front in North Africa, allied supplies to Russia etc.