He is revealed to be the killer in the final scene of the book, where he is waiting for Helen in her bedroom.
Here's a breakdown of why Max is the killer:
* He was present at the accident: Max was the one who drove the car that hit the boy, even though Julie was initially blamed.
* He was obsessed with Helen: Max had a stalker-like obsession with Helen and was jealous of her relationship with Barry.
* He was manipulating Julie: Max was manipulating Julie to protect him and his secret.
* He is the only one who knows the truth: Max is the only one who knows the full story of the accident and the subsequent cover-up.
The book leaves the ending open-ended, but it strongly suggests that Max is the killer and that he is going to attack Helen.