Amir experiences the realization of partial atonement for past sins while taking a beating at the hands of his old nemesis Assef. Amir knows that "There is a way to be good again," but it can only happen by successfully rescuing Sohrab from the Taliban. Assef, with brass knuckles and longstanding grudges, stands in Amir's way: It seems an impossible task, the passive Amir facing the bloodthirsty Assef; and when young Sohrab intervenes with the slingshot, the boy also experiences a moment of redemption when he disfigures his father's old tormenter. The pain is invigorating to Amir.
... for the first time since the winter of 1975, I felt at peace... I laughed... I hadn't been happy and I hadn't felt better, not at all. But I did now. My body was broken--just how badly I wouldn't find out until later--but I felt healed. Healed at last. (Chapter 22)
The beating symbolically serves as partial atonement for Amir, and the loss of Assef's eye fulfills an old promise made by Hassan. The scene can be considered an early climax to the novel, but Sohrab's improved mental health and relations with his father will still take some time.