By killing roughly 20 millions of its own citizens? By forcibly deporting the whole nations from their homeland to a different part of the USSR? By forcing the surviving rest into poverty? By enslaving other countries for half of the century?
I'm always surprised how unaware people in the West are of all of that (unlike Nazi crimes which are well known).
You might think that Hitler was the most evil person of all the time, but Stalin really tried his best to outdo him (and succeeded, some would say).
EDIT: Actually no, I'm not surprised that Western countries forgot about Stalin's atrocities. It was a deliberate choice - in order to win World War II Churchil and Roosvelt needed Stalin as an ally, but they couln't admit to their societies, to their voters, that they made a pact with the devil, so they made sure that Soviet crimes are forgotten.