In The Kite Runner, how are the concepts of betrayal and atonement developed through the relationships between characters? How does it illuminate the meaning of the work as a whole?

Khaled Hosseini's The Kite Runner is a novel about atonement. The narrator and protagonist, Amir, tells us from the beginning of the novel that he must make up for his "past of unatoned sins." When Rahim Khan, his father's friend and former business partner, calls him in the first chapter, Amir says that Rahim Khan wants him to visit Pakistan and Afghanistan again because "there is a way to be good again." So in the first chapter of the novel, the theme of redemption has already been introduced.
As we read chapters 2 through 9, we learn exactly why Amir feels he needs to redeem himself. As a child, Amir was close friends with Hassan, who (along with his father Ali) was a servant in Amir's home. However, the two grew up like brothers and feel a close bond. Hosseini has Amir tell us, though, that the boys are from two different ethnic groups and socioeconomic classes, and this makes all the difference in their relationship. Because Amir is in the majority group, he sometimes relies on his sense of superiority, especially when he feels like Hassan is better than him in some way. For example, when Hassan politely points out a plot hole in Amir's story, Amir hears a voice in his own mind tell him that he shouldn't have to hear criticism from a Hazara. Nonetheless, the boys spend time together and join forces in the annual kite-fighting tournament, with Amir as the fighter and Hassan as the kite runner. It is very important to Amir to win this tournament, as kite-flying is the one activity that bonds him to his father; he hopes to make his father proud by bringing back the biggest prize, which is the final kite cut down by the winning kite fighter. As it turns out, Amir does win and Hassan does run the kite down. However, when Amir goes to find Hassan, he discovers Hassan being assaulted by Assef and his friends in an alley. Amir does not help his friend and instead runs away. Although he knows he is a coward and is filled with shame, Amir again thinks that he is superior to Hassan and that can justify his inaction.
After the assault, Amir and Hassan have a very tense relationship until the point Amir frames Hassan for theft in an attempt to get him kicked out of the house. It doesn't go according to plan, though, as Baba (Amir's father) forgives Hassan (he didn't actually steal, and Baba probably knows that). However, Ali feels his son has been abused and mistreated too long, and they decide to leave. Amir never sees Hassan again.
Much later, twenty-six years to be exact, Rahim Khan calls Amir and asks him to come visit. When Amir arrives, he finds out Rahim Khan wants him to go rescue Hassan's son, Sohrab, from an orphanage and deliver him to an American couple to be adopted. Amir also learns that Hassan was Baba's son and Amir's half-brother, which makes Sohrab a blood relative. Amir must go through many obstacles, utmost of which is a showdown with Assef (now a Taliban leader). Through the process, Amir feels cleansed and like he is making up for his past wrongs. Hassan is dead, however, so there is no way to directly atone or obtain Hassan's forgiveness. Amir ends up adopting Sohrab and taking him to California, and he also must defend the boy against his father-in-law when he questions why Amir would take care of a Hazara boy. In the final scene of the novel, Amir is teaching Sohrab to fight kites, and it seems as though the situation might be looking up, but it will take some time for Sohrab to get over all of the trauma of his young life.
As I said, redemption is the central theme of the novel. Amir betrayed his best friend and must atone for his sins. He does so through the journey to save and adopt Sohrab. Readers may differ on whether Amir can be forgiven or can redeem himself for what he did (or didn't do) as a child, but it is clear that Hosseini wants to explore the idea that a traitor must seek redemption and that such atonement will not be easy.