Is it how many that makes the monster, or how quickly? In just 12 years, Hitler killed innocents at a rate of 104.6/hour! Stalin killed 71.3/hr. They were both terrible, but if Hitler had been in power as long as Stalin was, this wouldn't even be a debate. And if not for Stalin, he could have...
And there's also the fact that Stalin's Russia did more to destroy fascism than any other country, including the US. The Russians had beaten the Japanese so badly, that the Empire was probing Stalin for their own terms of surrender. Truman didn't want the Russians to get credit, so he nuked them.
Hitler is commonly taught in history classes as evil and gets a lot of attention in schools due to his leadership of the genocide of the Jews (and anyone who opposed his rule). Stalin is worse because of the sheer quantity of people he killed, most of them his own people.
Some would say that the end justifies the means. In which case stalin might have prevented more deaths than he contributed too in the end by his role in destroying fascism. We will probably never know.