Guilt and redemption are two of the primary themes of this novel, and Rahim uses Amir's guilt over his abandonment of Hassan during the attack and his subsequent treatment of Hassan shortly after that to pull him back to Afghanistan to search for Hassan's son, Sorab. Amir, as a first person narrator, has revealed to us from the first page of the novel that he feels guilty about what he did to Hassan back in their youth, and it is has all haunted him through to his adulthood. When Rahim calls to tell him about Hassan's death and the plight of his son Sorab, the past has "caught up" with the present of Amir's life. Though Rahim doesn't state anything directly, he makes it rather implicitly clear that he knows all that happened back on the day of the kite running contest and what Amir did to drive Hassan and his father from Amir and Baba's home. He also reveals that Hassan was Amir's half-brother, and therefore, Sorab, is Amir's half-nephew. Amir always sensed an unusual closeness between Baba and Hassan and his father, but this explains everything better. Amir is absolutely compelled to do what he can to do right by the memory of Hassan, to show loyalty to blood relations, and to right the wrongs of his past and achieve some measure of personal redemption and relief from the guilt he has carried around all these years. It is a wonderful irony that when he does find Sorab, Amir must fight the attacking bully of their past -- Assef, in order to secure Sorab's life. His physical fight with Assef is kind of the fight he should have had back when they were all boys. It is also a great irony that Sorab is the one to finally take down Assef with the use of sling-shot -- a weapon he learned to use from the master -- his father, Hassan.