[What follows is a conjecture, not something I’m absolutely convinced of.]
An alternate history in which Hitler survived and Stalin was defeated would be less bad than the history that we actually got, in which Stalin survived and Hitler was defeated.
Consider: Hitler consigned roughly 6 million people to death in his concentration camps. Stalin killed over 20 million people. On this basis alone, Stalin is a greater monster than Hitler. Hitler’s Holocaust is generally regarded as a greater crime than Stalin’s gulags because Hitler was targeting the extermination of a specific race, a genocide, while Stalin’s enemies were a much broader group. But now that I think of it, I’m not sure that “killed a wider variety of people” is really a point in Stalin’s favor.
Consider also that Stalinist Marxism was a deliberately international creed, which was successfully exported all over the globe, bringing mayhem, misery, and murder wherever it went. Many nations were turned from mildly disfunctional to horrific hellholes by the infection of Stalinism. (The dysfunction is necessary for Stalinism to get a foothold, but once it arrives Stalinism invariably makes things worse, not better.) Nazi Aryanism was a much more local form of insanity, which couldn’t really be replicated in other places or times. Fascism was never successfully exported outside of Europe, and it’s quite plausible that a Nazi Empire that occupied Eastern Europe would have been more benign than the Soviet Union.
The price of leaving Stalin in power was nuclear proliferation, the Cold War, and the long series of nasty conflicts all over the globe in which both the U.S. and the U.S.S.R. committed atrocities. We are still dealing with the echoes of those conflicts in the form of the interminable Israeli/Palestinian conflict, Islamist terrorism, and various Third World dictators, which were favored by the U.S. in earlier times as a stopgap against Communist expansion.
We have developed a national mythology that identifies Nazi Germany as a manifestation of Pure Evil which had to be eradicated at all costs, and we constantly remind ourselves of this fact by building monuments to the Nazi atrocities and killing the Nazis over and over again in movies and video games. Meanwhile Stalin’s Russia is regarded as an enemy, but an enemy that could be temporarily regarded as an ally, and who we could afford to oppose slowly over the next fify years rather than demolishing all at once in a hail of bombs. It is possible that this valuation has it backwards.
(Counterarguments: if Hitler had been allowed to live, it’s possible that he would eventually have racked up a body count as great as Stalin’s. And it’s very plausible that the U.S. would have fallen into a Cold War with Germany that was as costly and destructive as the actual Cold War.)