Chapter 7 in "The Kite Runner"
As much as I would like to, I don't hate Amir, I can't blame him for wanting to self preserve himself. Most people would disagree, and say that they would have "lept in to save him" or would have gotten help from other people. But what would a twelve year old you do? If you were watching your best friend being raped by the neighborhood bully, how would you react? Self preservation is the most basic instinct in the human race, it would be the same if you saw a mugging going down in an ally, you wouldn't step in to help, you would walk past and ignore it. The same goes for a rape, at twelve years old, you wouldn't step in to help the person. You'd be watching, or hiding, petrified of what you had just witnessed. Watching your friend at the age of twelve being raped is a traumatic experience for anyone. At any age watching your friend being raped would be traumatic. Although, granted we are older now and would think different, and would probably jump in to help, but at twelve, the mind is still innocent and developing. Amir dealt with the situation in the only way he knew how: preserving his own life at the cost of his best friend being raped. Amir has it rough too, he has to live with the fact that his decision led to the rape of Hassan.
Even if Amir did step in, what would that change? Assef and his friends still had Hassan pinned to the ground, so he was a little useless, and even if they got up, it would be 3 big guys against a little Hazara and Amir. If Amir went and got help, Hassan would still be raped, but Assef would probably be finished by the time he came back and there would be no proof that the rape ever even happened, it would be word against word. Either situation is a loss for both boys.
Amir compares the look on Hassan's face to that of a lamb, and then recalls a somewhat brutal ritual of the killing a lamb. This is a metaphor for how the lamb in essence is Hassan in this situation, useless help its own demise. The lamb metaphor is in someway also connected to Amir, because as he watches this ritual of killing the lamb, he also watches the rape happen because of the same reason he watches the lamb being killed.
In conclusion, I don't think its fair to blame Amir for the decisions he made, because he is young and, because everyone could or would handle that differently and in there own way.