What?!? No, what people are saying is that: first, it would have been much riskier for Hitler to have invaded Poland without the pact with Russia, so the pact made war more likely. This is a fact, which has nothing to do with "free will"... just common sense. And secondly, as soon as Poland was crushed in the West by the Wehrmacht, the USSR took advantage of this to invade and conquer part of a sovereign nation. This is also an indisputable fact.
The Russian argument is a little bit like arguing that, after watching a passerby get mugged, beaten senseless, and robbed of his wallet, it's also OK for you to step in to steal the watch off of his arm as he lies there unconscious. Sorry, no one with any sense or morality would agree...
If only the Russian argument were as convincing as that.
But the real analogy would be: Hitler tells you he's going to kill a guy and invites you to join in. You don't jump in right away because you're not sure how tough he is, and besides, your own army isn't really very good (yet.) Seeing things are going well, you decide to jump in, take some booty, and announce, by the way, that your arrangement with him entitles you to kill a couple of cripples over at the hospital named Yugoslavia, Romania, and the Baltics as well.
It's darkly hilarious that Neville Chamberlain (and to a lesser extent Clemenceau) have taken such a beating over the years. No one has more responsibility for empowering Hitler at the time of his greatest weakness than Josef Stalin. No one.