"That's why, however justified the critique of Stalin, I just can't abide the whole line that WWII was just a conflict of "inter-imperialist rivalry"."
This is a non-sequitur, and thus a very bad way to resolve this type of question. Whatever your view of the USSR, its participation in the war against Nazi Germany does not confer on it any more socialist credentials than the capitalist democracies with whom it allied.
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Well, there's a larger issue here - in the counter-factual without an October Revolution and a USSR, there almost certainly would not have been a National Socialism either. Without Nazism, indeed without Hitler in particular, would there have been another war in Europe? Certainly, imperialist Britain and France didn't seem keen on another war. Nor did America.
In the end, we can give credit to the USSR for heroically defeating a menace that only existed in the eventuality that a Bolshevik Revolution happened at all. This is the rational kernel of the reactionary position of people like Ernst Nolte in the historian's controversy that occurred in Germany in the late 1980's.