I think that it was after the rape that Amirs guilt truly took over, that was the downfall of their friendship. After the rape, they lied abut what happened, then Amir with the pomagrantes thrown at Hassan. That's when he realized that he needs to get rid of Hassan to get rid of that guilt, but it backfired and they still left. But I connects with the end with Amir feeling that he needed to go save Sorhab to make up for everything he did to Hassan. It was a necessary journey for Amir to make to try to mend their friendship on his part.