Sometimes the reason is quite complicated. In my case it isn't. I simply enjoy seeing the hero/protagonist, the strongest and best resource the good guys have, be absolutely lost both in mind and body with no chance of turning back. Ending as the very thing that helps evil win with zero hesitation or remorse. An absolutely delicious bad end.that fetish is a psychoanalytic goldmine
Like when Claire wants to stop Overwatch Widowmaker from taking over the lands but gets captured, resisting till the poison starts to corrupt her utterly. That was some nice shit.