Amir constantly has been remembering the events of him having had ran away after watching Hassan experience SA. This has led to an overwhelming amount of guilt, and he doesn't exactly know where to channel that guilt. In an attempt to get rid of said guilt, he hits Hassan with pomegranates in order to get Hassan to hit him back. This is because in Amir's point of view, he feels as if he deserves punishment for what he had done. Punishment in Amir's case in this position is a significance of relief from guilt.
Hit me back!
The pomegranate juice has the visual resemblance of blood, and blood can be looked at as a result of violence. Amir wants Hassan to hit him back with the pomegranate, which can be visualized as violence. Said violence can be correlated to punishment, so the redness of the pomegranate juice resembling blood could be seen as Amir craving redemption by punishment to get rid of this guilt he feels.
There. Are you satisfied? Do you feel better?
Though, as seen earlier, the pomegranate juice is mainly smeared over Hassan, and Hassan is seen hitting himself instead of Amir, showcasing how loyal Hassan is towards Amir as he is putting Amir's perception of a punishment, in this case, on himself. This can lead to Amir feeling even more guilt as he hadn't been loyal towards Hassan when he ran away after seeing Hassan experience SA, but Hassan has continued being loyal to Amir no matter what has happened.
What am I going to do with you, Hassan? What am I going to do with you?