I think this is a great question with the possibility for many excellent answers. The previous post certainly hit on one concerning Amir's failure to help Hassan when he was being sodomized by Assef. I think one of the reasons that Amir made the return trip to Pakistan to see Rahim (and, later, to Afghanistan) was in the hope that he could reunite with Hassan. I'm sure Amir would have loved to apologize to Hassan for his past indiscretions, and this simple act would have helped Amir relieve his longstanding guilt. Meeting Hassan and his wife and son would have been a great reunion for Amir; unfortunately, he soon discovered from Rahim that this would be impossible: Hassan and his wife had already been killed by the Taliban.
What precisely are you refering to here? It is very hard for us to second guess characters and authors and talk about what should have happened in a novel, as obviously authors have worked very hard to plan and plot their story so that what happens feeds in to the overall character development and plot outline. What I think you are refering to is in Chapter Seven, which of course narrates the tragic scene when Hassan is raped and Amir stands by and does nothing. This of course represents a major turning point in Amir's life, as his decision will profoundly affect the rest of the tale and his own character development. Note how this is indicated:
I had one last chance to make a decision. One final opportunity to decide who I was going to be. I could step into that alley, stand up for Hassan--the way he'd stood up for me all those times in the past--and accept whatever would happen to me. Or I could run.
It is very easy for us to say that Amir should have stayed, but in a sense, his decision to run leads up to his final decision to face Assef in the future--a kind of echo of this scene, where Amir is given the chance again to make the right decision, but this time to save Hassan's son. Thus in this chapter we see that Amir makes a decision that may be "wrong," but nevertheless plays a profound part of shaping his identity and the rest of his novel as Amir searches endlessly for an opportunity to redeem himself and finally discovers it.