This, because it's what Nick agonizes over and wonders about all the time. Doubles as a Crowning Moment of Heartwarming.
Anzu: He left you, you know. Your precious brother. We don't lie. You know I am telling the truth he never did. He left you a thousand times. He used to lie in bed daydreaming about he and his father driving off, getting away from you when you were a nightmare child with black button eyes. He used to not be able to sleep because he was scared of you! He worked with his leg hurting, and he thought about how much easier the struggle would be if he didn't have to feed you and your brother. He knew Mae preferred you, so many girls preferred you, and he resented you for that. He would get in the car and drive away and leave you for ten, fifteen minutes, driving out of the city never to come back, until he turned round. He meant to leave you. You took his life, and you took his chance at love, and he hated you, and he wanted to leave you!
Nick: But he didn't leave.
Anzu: No. But he wanted to. He should have. If he had, he'd still be alive, wouldn't he? I didn't kill him. You did. He would have lived, without you. He would have had a life, if only he hadn't wasted his time trying to love something that could never love him back.
Nick: Who knows? Maybe I did.