I think it’s more accurate is to say that they were on the good side (allies), but they had their own imperialistic interests. Well, and also committed a lot of war crimes.
So yeah, they were the bad guys, just on the other side.
They literally signed a public pact with the Nazis and a secret pact to split Europe up, before the Nazis invaded them. They had a common enemy in the “West” and plenty of European lands (Estonia, Latvia, Romania, Finland, Lithuania) to split in between.
Stalin got greedy though and decided to invade a part of the then Romania outside the sphere of influence that had been agreed in the secret pact. It’s not clear if the Nazis would have launched Barbarossa anyway, but Stalin claiming lands beyond their agreement in their eastern flank didn’t help.
They were hardly “good guys”, invading sovereign lands and imposing terror, they just had to fight against the Nazis when it turned out the Nazis invaded them before they finished fighting the West.
