"Seriously, do we really have to drive home the point how horrible things would've turned out if National Socialist Germany had won the war?
"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. The question of whether the war was *in the last analysis* an inter-imperialist rivalry also does not pivot on whether the victory of one side would have been more or less catastrophic for human survival. Few would query the idea that Nazi Germany represented the apex of imperialist barbarism, and that it posed a far more acute and immediate danger to humanity than its rivals: but that does not mean that its rivals were not driven by imperialist competition. The status of the USSR can simply not be resolved by this sort of argument, which is ultimately an appeal to sentiment.