If the above is not what you meant by your question, let me know. After Rahim Khan pressures Amir to return to Afghanistan and tells Amir that Hassan was his half-brother, Amir knows that going back to his homeland to rescue his nephew is the only way that he can come close to redeeming himself for not saving Hassan so many years ago.
Of course, Assef, the ultimate villain, returns for this portion of the novel. Assef creates several obstacles for Amir. Practically speaking, he lures Amir to a rather remote location that isolates Amir from help if he should need it. Assef knew who Amir was all along. He could have easily talked to Amir in the stadium.
Secondly, Assef taunts Amir with Sohrab to let Amir what he had put the boy through. He strokes Sohrab in front of Amir and forces him to perform a humiliating dance, all of which hints at the torture Assef has forced Sohrab to face.
The most significant obstacle is that Amir must face his cowardliness from so many years ago. If Sohrab's captor were just any member of the Taliban, the rescue would not have the importance that it did. By having to go hand to hand with Assef, Amir must finally give up being intimidated by the bully. Likewise, by rescuing Sohrab from Assef, Amir is given the opportunity to right the wrong from his past. It is not a "clean" fix though. Even if Amir had killed Assef, Assef's obstacle of molesting Sohrab is something that Amir and Sohrab will have to repair for years to come.