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...

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.