What is a passage in Khaled Hosseini's The Kite Runner that shows Amir's guilt for what he let happen to Hassan?

As the central conflict of The Kite Runner is how Amir deals with and eventually works to redeem his guilt following the assault of Hassan in Chapter 7, there are numerous passages that could serve as good examples. I will list a few and discuss briefly how they relate to the central conflict. 

1. In Chapter 8, on page 86, Amir says: 

"I watched Hassan get raped," I said to no one. Baba stirred in his sleep. Kaka Homayoun grunted. A part of me was hoping someone would wake up and hear, so I wouldn't have to live with this lie anymore. But no one woke up and in the silence that followed, I understood the nature of my new curse: I was going to get away with it.

I thought about Hassan's dream, the one about us swimming in the lake. There is no monster, he'd said, just water. Except he'd been wrong about that. There was a monster in the lake. it had grabbed Hassan by the ankles, dragged him to the murky bottom. I was that monster. 

That was the night I became an insomniac.

This passage shows Amir wishing he could get this secret—that he witnessed the assault but did nothing to help Hassan, running away instead—off his chest to assuage his guilt. He returns to the dream Hassan told him about before the kite tournament and revises Hassan's interpretation that there was no monster. Amir's claim that he, Amir, is actually the monster dragging Hassan down is a vivid metaphor that captures the depth of Amir's guilt. He knows he has betrayed his friend and sees his inability or unwillingness to help Hassan as just as pivotal to Hassan's pain as Assef's rape of Hassan. Amir ends this passage by noting that he can no longer sleep peacefully because of the psychological burden he feels.

2. Another scene that displays Amir's guilt occurs later in Chapter 8, when Amir screams at Hassan to throw pomegranates at him. This scene takes place on pages 92-93. In the middle of the scene, Amir says, 

I wished he'd give me the punishment I craved, so maybe I'd finally sleep at night. Maybe then things could return to how they used to be between us (92).

Amir wants to be punished for his disloyalty, but Hassan cannot do this. Even if it had been consistent with his personality (it's not), Hassan's position in the household as Amir's servant and as inferior in terms of ethnic group, religion, and class, mean he cannot outwardly express anger (verbally or physically) toward Amir for Amir's betrayal. In the scene, Amir is obviously projecting his guilt onto Hassan. In response, Hassan smashes a pomegranate on his own face and asks Amir if he is "satisfied" (93). It is after this scene that Amir decides to place the watch and birthday money under Hassan's pillow to frame him for theft so Baba will send Ali and Hassan away.

3. Another scene later in the novel that captures Amir's continuing guilt is Amir's description of a dream he has after learning of Hassan's death. The dream is described in Chapter 19, pages 239-240, in a series of italicized paragraphs. Amir imagines the moment of Hassan's death as it was relayed to him by Rahim Khan in the previous chapter: Hassan is kneeling in the street and is shot in the back of the head. At the end of the dream, Amir reveals that he is "the man in the herringbone vest" who is behind Hassan and who shoots him with the rifle. In reality, someone from the Taliban shot Hassan, but after Amir has heard of all the tragedies that befell Hassan while he was still in Kabul (and Amir and Baba were in California), he feels responsible for everything, including Hassan's death. This recalls the earlier passage where Amir sees himself as the monster in Hassan's dream.

All of these passages and many others throughout the novel show Amir's guilt and also support the necessity of the journey he takes to redeem himself and atone for his sins later in his life.