You seem to misunderstand who controlled USSR then.
Stalin coopertaed with Hitler a lot. Stalin bought a lot of military technology and outright weapons from Germany (such as cannons). Stalin and Hitler collectively destroyed Poland in 1939, divided the territory, and had a parade where Wermacht and Red Army troops marched together.
At the same time Hitler planned to attack USSR, while USSR secretly planned to attack Hitler.
Before that, Stalin robbed Soviet peasants of the wheat they produced, and sold it all for gold, to buy weapons factories, mostly from the US. In 1933, this resulted in famine in Ukraine and some other parts of USSR (google Holodomor).
No way one could expect Stalin's regime to soften down and become nicer.
Certainly, Soviet soldiers who fought against Hitler, along with other Allied soldiers, fought for good, or at least for a better world. But these same soldiers, who liberated countries like Poland, or Hungary, or half of Germany from Hitler put these lands under control of USSR, not direct, but pretty stifling. They suppresses attempts to get out of Soviet control in Hungary and Checho-Slovakia with tanks soon after.
