In the novel The Kite Runner, why did the author have Assef, Sohrab, and Amir all come together later on in the story?

At the beginning of the novel, Assef challenges both Amir and Hassan on their way to climb a tree. Hassan ends up saving himself and Amir by pointing his slingshot at Assef and threatening him. Assef, disappointed at the fact that Hassan is armed says, "This doesn't end today, believe me...This isn't the end for you either, Amir. Someday, I'll make you face me one on one" (Hosseini 35). Shortly following their controversial interaction with Assef, Assef corners Hassan at the end of a kite-fighting tournament and rapes him while Amir watches at a safe distance. Amir is plagued with guilt for the majority of the novel until he is given a chance at redemption.
Later on in the novel, Amir is older and embarks on a mission to save Sohrab, who is essentially a sex slave to Assef. Sohrab, Hassan's son, is very similar to his father and Amir has to fight Assef one on one in order to leave with Sohrab. Khaled Hosseini chose to bring Assef, Sohrab, and Amir together in Chapter 22 in order to emphasize Amir's chance at redemption while creating a circular narrative. In dramatic fashion, Amir conquers his fear of Assef by fighting him behind closed doors in front of Sohrab and gains redemption by doing so. Sohrab, who represents Hassan, saves Amir by shooting Assef's eye out using his slingshot. The way in which Sohrab saves Amir resembles the way Hassan protected Amir in his youth. Hosseini ties together two significant scenes by having essentially the same characters, engage in similar confrontation.