It has to be my favorite archetype: the corruption of innocence, where a person slowly loses their sense of clarity or goodness and slips into madness. What makes it interesting is how gradual and believable the change feels. In the process of fighting something they see as wrong, they begin to adopt the same traits and methods they once opposed. Over time, they become indistinguishable from it, losing the very values that defined them in the beginning.
As Friedrich Nietzsche put it:
"He who fights with monsters should look to it that he himself does not become a monster. And if you gaze long into an abyss, the abyss also gazes into you."