"Why is it," asks my inquisitive son as we walk home from school, "that people don't think Josef Stalin is as bad as Hitler when actually he's much worse?"
"Well, son," I say. "It has to do with the fact that Hitler is fascist--which they think is right wing--and Stalin was a Communist, and therefore on and of the left. So he gets a pass because, being on the left, he meant to do good, even if it didn't exactly turn out that way. Also, he ended up being our ally against Hitler (after signing that pact with him earlier in the war)."
Having read millions of words on the subject of both dictators and their times, that's the best explanation I come up with on this, the 60th anniversary of Stalin's death.