How's this for a hot take?

Consuming dark fiction, though it allows the audience to empathise with morally dubious or outright evil characters, nonetheless increases their overall amount of internal empathy towards other people. Not just bad people benefit from an overall increase in empathy.

Consuming exclusively sanitised, fluffy feel-good fiction with no counterbalancing dark sides does not increase the audience's empathy, because anyone who cannot empathise with The Littlest Cinnamon Roll, who has adventures dealing with happiness and friendship and harmony, is already a lost cause. Rather, such fiction, when all other kinds are excluded, normalises the idea that the only people worth empathising with are the morally pure cinnamon rolls who have never done anything wrong in their entire lives. The normalisation, and reinforcement of same through the standard life cycle of a fandom, reaches such a point that as soon as the characters in question step outside the boundaries of that bubble of innocence by doing (1) problematic thing, they are cancelled for crimes against humanity and the fandom tears itself apart.

This is my explanation for not only why fandoms for fun kids' shows tear themselves apart over the slightest perceived creative slip-up, but also why the most passionately engaged fans of these things (who more often than not precipitate the rot/collapse) tend to be such toxic people in all other spheres of life as well.