The response of Amir

Amir response Farid like that means that he think the ending for him or for Assef is what they should get. He think that they both are sinner.
And I agree with what Amir said. In his mind,what he deserved is to pay back what he haven't done. But Hassan's death brought away the chance. What happened in the house made him faced what he have ran away when he was young. He think this is what he should deserved. Maybe it's a kind of way to expiate his sins.

To Assef, he is really a sinner. The ending is what he should get. What goes around, comes around. He really got what he deserved.