“portray it if not as the main culprit of the war, then at least as an aggressor, along with Nazi Germany”
They were an aggressor, and an ally (secretly) of Germany at the start of the war. It was no coincidence that Russia invaded Poland at exactly the same time as the Germans invaded Poland, and it was no coincidence that they split the country in two and never fired a shot at each other.
“equate Nazi occupation, which claimed tens of millions of lives, and the crimes committed by collaborationists with the Red Army’s liberating mission”
The Red Army liberated nothing. All the territory it occupied was installed with puppet governments and remained enslaved to the Soviets for the next thirty odd years. They also slaughtered plenty of civilians (and surrendered prisoners of war) in the course of their occupation.
They murdered one of the biggest unknown heroes of the war.
Witold Pilecki
https://en.wikipedia.org/wiki/Witold_Pilecki
Another victim of the Communists:
Milada Horakova
https://en.wikipedia.org/wiki/Milada_Hor%C3%A1kov%C3%A1