While undoubtedly true, one should keep in mind that Stalin first cynically entered into a non-aggression pact with the nazi regime, showing blatant disregard for Poland in the process.

Not to mention that a significant part of the Soviet Union's losses in WW2 came about from a similar disregard for the lives of its own soldiers.

This is not to discount the sacrifice made by those who fought and died, though.

(And Treptower Park is, as you say, hauntingly powerful. I've been there on three occasions when visiting Berlin.)