Soviets didn't like Polish and wanted to make good commies form this nation. They expelled Polish form east but from the other hand made some space for Polish on west.
Germans wanted to exterminate all Polish.
Lukasz no one is arguing that Soviets were equal to Germans, the point is however that ultimately Russia was an enemy of Poland and there can be no talk of brotherhood in arms because the red army was ultimately an invading force that just happened not to be THAT bloodthirsty.