How does Amir treat Assef later on?

As an adolescent, Amir goes out of his way to avoid Assef and absolutely despises him. Assef is the neighborhood bully who strikes fear into Amir and the other local boys in Kabul. Amir describes Assef as a sociopath and passively watches as Assef rapes his best friend, Hassan, after the kite-fighting tournament. Amir refuses to intervene and help his friend because he fears that Assef will also harm him. Amir's fear and jealousy stand in the way of helping Hassan, who is defenseless against Assef and his two friends.
As an adult, Amir attempts to find redemption and atone for his past sins by traveling back to Afghanistan and saving Sohrab from a difficult life of abuse and poverty. When he discovers that he is speaking to Assef, who is now a leading Taliban official, Amir is both shocked and afraid. Amir initially offers to pay Assef in order to leave with Sohrab, but Assef challenges Amir to a duel. He tells Amir that if he defeats him in a hand-to-hand fight, he will allow Amir to walk out the door with Sohrab. Instead of running away and avoiding Assef like he did as an adolescent, Amir faces his fears and accepts Assef's challenge. Amir suffers a brutal beating but manages to survive and leave with Sohrab after Sohrab knocks Assef's eye out using his slingshot. Overall, Amir treats Assef differently as an adult by standing up to his enemy and engaging in a violent fight to save Sohrab.