That’s exactly the point. Had the Nazis won WWII, the narrative would have been the allies were the one that commit genocides, the morally corrupt, and the war-mongers; the Nazis will boast themselves as the most righteous, and most people would have no problem believing that. Hard to imagine for sure, but in a world where the winners write the history, it would be naive to think only the winners must be just because “justice always prevails”.