What precisely, is the "sin" of which Amir is guilty? What are the various consequences of his belief that he has committed an unforgivable sin?

To be more precise, Amir committed several terrible acts against his servant "friend," Hassan, in Khaled Hosseini's novel, The Kite Runner. It is true that Amir never fully embraced Hassan as an equal or as a friend or as a member of the household. These are additional guilts with which Amir has had to live. But the two specific acts for which Amir feels most guilty concern (1) Hassan's rape at the hands of Assef; and (2) Amir's planting of money and a watch in Hassan's room and then claiming that they have been stolen.

When Amir follows Hassan as he chases down a kite, Assef and his mates corner Hassan. Amir watches from a distance as the boys hold Hassan down while Assef rapes him. Amir could have come to Hassan's aid, but his timidity causes him to stay silent. When he confronts Hassan later, he pretends to know nothing of the rape, and Hassan never admits to it.

Later, after Baba has paid for an operation to correct Hassan's hairlip, Amir's jealousy gets the best of him. He takes his watch and money (birthday presents) and plants them in Hassan's room. He then tells Baba that they have been stolen. When they turn up in Hassan's room, Hassan admits to stealing them. Baba forgives him, but Hassan and his father, Ali, leave the home in disgrace. Amir can never forgive himself for these two acts.