“No! I must kill the demons,” he shouted!
The radio said “No, John. You are the demons.”
And then, John was a Zombie.
Yeah, these are the kinds of fanfics I read.
In seriousness, there was this one fanfic series that had an alternate dark ending where the main character fails to escape a timeloop, goes insane, carves her childhood friend’s heart out and eats it, and becomes a corrupted insect monster, ruling over a post-apocalyptic timeline.