What causes Tony to torment and bully Lani in the novel, What Happened to Lani Garver?

Tony Clementi is a closeted homosexual, who torments and abuses Lani as a way of punishing himself for his own shame. He tries to molest Lani outside a bar one evening, and when his friends approach, Tony swears that Lani was coming on to him. From then on, Tony is on a mission to torment Lani.