It's the same here in germany, while you can't say Hitler and something not negative in the same sentence, you can freely worship the likes of Stalin, Mao etc.
Concerning Hitler I'd say the reason, he is that much hated, is because he was so full of hate and didnt seem to have a reason for his killings. It's natural to fear what you don't understand.
On the other hand, Stalins murders seemed understandable, at least to some extent.
I also think that it has to do something with the fact that the Western Allies thought they were able to control Hitler, but failed misarably.
Stalin on the other side took part in the game until 1945, the leaders of the West thought they could handle him. Well, here again, they utterly failed.
Ask the Admissions Team anything!