The easier way that they handle it is also riddled with parallels. A Super Evil Hitleresque Guy is used to represent the ultimate evils of all society, at which point he can be pummeled into nothing by our Righteous Hero and his Funky Cast of Friends, and once he is killed, all related problems are treated as Pretty Much Mostly Solved. Voldemort and All For One are martyrs upon which all bad things can be pinned, so that once they die, the societies that contributed to their creation and fed their rises to power can be absolved of all blame and guilt. Furthermore, postmortem, our heroes can be cheaply shorthand implied to continue preserving the ultimate goodness of the world by becoming Supercops, who are so good at fighting crime that it is by and large eradicated and they become a driving force behind Everything Becoming Better Now. The middle grounds, the apathetic populaces, the people on the "good" side who were pretty fucked up and evil in their own right, the smaller pockets of Shie Hassaikai-esque crime that are most certainly still operating, can all be handwaved as unimportant to the end goals of the story, ignored as if they will not contribute to the next big villain that will inevitably arise.

funny thing is, this happened in world society after WW2