Slate has a column inspired by a new book on Hitler and Stalin. Part way through the column the author wonders why we remember Hitler's evil but overlook Stalin's? After all, Stalin killed several times as many people as Hitler (Stalin killed 100 million while Hitler killed 25 million). I can think of several factors.
It is important to remember that neither came from a vacuum. During the 1930s both had their supporters among American intellectuals. Fascism was promoted as the third way, half-way between communism and capitalism. Both Hitler and Stalin were providing direct support to their followers in the US. Hitler got support from unexpected sources. W.E.B. Dubois visited Nazi Germany and came away impressed with how efficiently the government was run (while shocked by their anti-Semitism). He was also a huge supporter of Imperial Japan.
Hitler's genocide was the dark side of the eugenics movement which was also popular among the intellectuals. Margaret Sanger, the founder of Planned Parenthood, was a believer in the softer side of eugenics - where Hitler wanted to eliminate the undesirables, she simply wanted to keep them from breeding.
The big difference is that we went to war with the fascists - a total war where most of the nation was mobilized and nearly everyone had friends and relatives in the fight. This discredited fascism. Once the bombs dropped on Pearl Harbor, no one wanted to be associated with it any longer. It went from mainstream to fringe overnight. Further, once we won the war we saw the concentration camps full of skeletal survivors and we got our hands on the records showing how many were gassed and incinerated. We were properly horrified.
This never happened with Stalin. He was an ally during WWII so the US government had an interest in suppressing stories about communist atrocities. The wars we had with communist countries were all smaller and never shocked the nation the way that WWII had. There was no mass conversion of communists the way that we had with fascists.
We never invaded the USSR and saw the results of Stalin's brutality with our own eyes. Stalin was good at suppressing information and there were plenty of communists and communist sympathizers in the US who were sure that nothing bad could possibly happen in the worker's paradise. They dismissed reports of mass starvation as propaganda.
Stalin was dead for decades before the truth came out. Even now there are apologists who say that Stalin was trying to accomplish a greater good so the deaths of 100 million were justified by his goals.
In the decades since Hitler's death, our image of him has changed. I have seen that over my lifetime. Our image of him has gone from being the worst of a crop of evil dictators to being the epitome of evil.
A few years ago some color movies of Hitler came to light. Some of them were taken by his wife. They show Hitler at ease, smiling and joking. There was real concern about showing these because they conflicted with our image of him as evil personified. Similar arguments have been raised about evaluating Stalin's evil. There are suggestions that this is really an attempt at rehabilitating Hitler.
There are important lessons to be learned here. The most important is that Hitler was not unique. Other leaders have risen to power and killed millions in the name of an ideology. A related lesson is that ideology does not excuse evil. Stalin killed tens of millions. Pol Pot "only" killed a million or two but it was still one out of every five in Cambodia. Deaths under Chairman Mao were somewhere between 20 and 70 million. All of these people were killed in the name of ideologies. If they died of starvation it is because the food was forcibly taken from them.
The final lesson is that many of the ideas that gave rise to these evils are still present in our culture today. They came about because the intellectuals of the early 20th century decided that they knew better ways of structuring society. If the were just given control they would solve the problems of the world. To paraphrase Mao, if half the country had to die to achieve these goals it would be worth it.