Say you've made some new friends who are a little rough around the edges, and you're hanging out with them. Suddenly, they start speaking ill of someone you know, not just anyone, but the girl you eventually start to develop feelings for. You confront them and call them out, only to be shot down with the revelation that they used you to get close to her and hurt her. Poor Mike...
Evelyn has it bad. Very, very bad. To find out that your best friend's crush on you is the reason why people are dropping dead like flies is downright frightening and traumatizing.
Actually, the whole final arc is made of nightmare fuel. It was mentioned that the main characters are only sixteen, and for them to go through all of that...
This exchange pretty much sums it up:
Matt: How have you been, Mara?
Mara: I've been okay. Well, as okay as someone can be when they figure out their friend was a psycho.