What does Amir mean when he tells Farid that both he and Assef got what they deserved?

Assef was a sociopath murderer who at least got his eye taken out by Sohrab's slingshot. Amir had been waiting for a beating from Assef for most of his life. Amir feels he should have taken the beating by protecting his friend/brother Hassan so many years ago. The guilt weighs heavy on Amir almost incapacitating him. The beating he finally took from Assef was cathartic: Amir welcomed it as long overdue.