The Taco Shack Killer (Unabridged‪)‬

The Taco Shack Killer is a book about a boy who is bullied as a young teen, and uses his anger to kill the kid that is doing the bullying. As he grows up, every time he runs into a bully in his life, he kills them. 

As an older teen, he gets a job at the Taco Shack, and goes insane working a job he hates, killing off most of the crew and some of the management.