5 Reasons Gabe believes he’s a monster:
It goes back to his childhood (which is a major spoiler that I can’t explore here. You’ll have to read the book)
Gabe wants to retaliate with all of the bullying, but he’s afraid of how far he could go with his anger. This also comes back to Seth (The Ugly Truth) and what Gabe’s believes about justice.
During the 8th grade, Gabe beat up someone he considered a friend - Cord - and lost control of his anger. This has been a formative reason in holding what he believes to be the monster back.
Gabe isn’t violent. He’s sensitive and silent, but it also makes him dangerous because he knows where that kind of dormant anger leads.
He hurts everyone he loves (at least that’s what he believes about himself).