Final answer:
Assef's parting remarks after losing to Hassan in "The Kite Runner" included a vow to take revenge on Hassan and Amir in the future.
Step-by-step explanation:
The character Assef in Khaled Hosseini's novel "The Kite Runner" is known for his antagonistic and violent behavior. After losing to Hassan, who defended Amir with his slingshot, Assef vowed to take revenge on both Hassan and Amir.
Despite the immediate defeat, Assef's parting remarks were filled with underlying threats, hinting that he would not forget the incident and that they would pay for their defiance in the future.