Amir's Betrayal of Hassan
Even though Hassan is a steadily loyal friend to Amir, Amir betrays Hassan multiple times throughout The Kite Runner. The betrayals start out small, like in Chapter Four when Amir reads to Hassan and when Hassan asked what big words meant, Amir would "tease him, expose his ignorance," though he "would always feel guilty about it later." The more life changing betrayal occurs in Chapter Seven, when Amir watches Assef rape Hassan and does nothing to stand up for him, choosing instead to run away. In Chapter Nine, Amir betrays Hassan again when he frames him for stealing Amir's watch and birthday money. He does this because he feels guilty about his previous betrayal, wanting "to move on, to forget, to start with a clean slate." All it does is make his guilt even worse though.
Baba's Betrayal of Ali
Toward the end of The Kite Runner, it is revealed that Baba is another character who has betrayed someone. When Amir goes to Pakistan to see Rahim Khan, he finds out that Baba slept with Ali's wife, and that he is the true father of Hassan. On page 226, Amir confesses that this realization shows that he and Baba "were more alike than I'd ever known." Each betrayed the one person in their lives who was most loyal to them. In a way, Baba also betrayed Hassan because he lied to him and kept him from knowing who his true father was.