Well, they may have rationalized it in that way (I have no doubt they did), but the fact is that the Russians negotiated the partition of Poland BEFORE the Germans had even attacked it.
The Russians were nobody's savior.
I believe Germany could have defeated Russia if only they had treated the Russians with just a smidgen of Humanity......Originally they were welcomed as liberators from those who were happy to be out from Stalin’s jackboot. Even had a high ranking Red Army general, Vlasov, offering his services to fight against the Bolsheviks.
I believe it would have been very easy for Hitler to overthrow the Bolshevik regime, I think Stalin was toast if the Nazis managed to take Moscow....of course, perhaps Hitler understand his replacement could have been even more formidable.
True, they just traded one set of chains for another.