The physical scar that Amir gets in his fight with Assef is significant for a couple of reasons. The first is that it shows sacrifice. Amir is finally sacrificing for his friendship, the way he should have when he saw Ali being violated in the alley after the kite festival. The sacrifice he has to make now is the result of years of repression and denial. His scar is also significant because it moves him closer to Ali, as his hare lip before the surgery defined him and Amir's scar now will help to define him. Amir's physicality cannot be removed, as it has become a part of him, representation of the commitment both he denied and eventually embraced. In the notion of "becoming good again," the scar goes a long way to that point. I would also say that the scar is representative of the physical notion of taking action. Part of Amir's guilt and shame resides the fact that he did nothing when he saw Ali being violated. The physicality of action is something that is represented by the scar. This is beyond words, beyond thought, but rather pure action, representing a commitment towards that which is good.