This scene represents the theme of betrayal, as Amir watches his dedicated, very loyal friend Hassan get raped, and does nothing to stop it. Amir describes Hassan as a lamb at the slaughter, helpless and innocent, yet destined to be sacrificed. In this case, Hassan's well-being is being sacrificed so that Amir may finally receive his father's love, via the blue kit Hassan had retrieved.
Amir then flees, instead of intervening to help Hassan, further cementing his status as a guilty party. By running away, Amir shows that the value of Hassan's life has decreased in his eyes, despite having grown up with him and being practically brothers, having asked himself, "Was it a fair price? ... He was just a Hazara, wasn't he?" (Hosseini, 77).
After finally finding Hassan again, Amir pretends not to have witnessed Hassan's rape, a final display of the theme of betrayal. This was Amir's opportunity to redeem himself by comforting Hassan or at the very least thanking him for running the kite, but all Amir does continue with his act of not knowing. The unmentioned blood dripping down Hassan's leg as they walk back home represents the guilt and embarrassment both Amir and Hassan feel after the assault, unable to be controlled, held back, or hidden, yet still ignored.