Why does Amir recalls the memory of the lamb being sacraficed when he witnesses Hassan's tragedy in the alleyway?
Amir recollects the memory again toward the end of the novel when he sees Sohrab in the home of the Taliban. I also don't understand how to discuss the image in the context of the novel.?
I have a little different take on why Amir's mind flashes back to the sacrificial lamb during the attack on Hassan in Chapter 7. At this point in the novel, Amir is at a crossroads--he so desperately wants Baba's approval, that he must decide what he is willing to sacrifice to get it.
As he comes upon the scene in the alley, Amir knows that Hassan is so loyal to him that he will sacrifice himself to keep the kite for Amir. However, Amir is actually the one in charge of the sacrifice; he could choose to step in or call for help, but he does neither. Instead, he makes the conscious decision to sacrifice Hassan's innocence and peace of mind so that he can obtain Baba's approval.
The image of the literal lamb being led to the sacrificial slaughter is something that always horrified Amir, and so his memory of it in Chapter 7 demonstrates that Amir knows just how harmful his choice to sacrifice Hassan is. Hosseini includes this flasback on purpose; so look for other examples in The Kite Runner of Amir's dreams or memories illuminating his guilty conscience.
Well, it is a pretty common image, the idea of the sacrificial lamb, and runs through many religious and philosophical systems.
In this case, it is simply the fact that Hassan goes to the slaughter willingly, as a lamb. He runs in to get the kite knowing what might happen.
The second part of it is the helplessness of the lamb in both Sohrab and Hassan's case. If no one steps in to save it, it is obviously powerless against the slaughtering person or thing. So too are Hassan and Sohrab, but in the latter case, Amir finds the courage to try to step in and do something about it, to save the lamb from the slaughter.