why is hssan considered a sacrifical lamb?

Amir turned away, weeping, still hearing Assef's grunts issuing from the alleyway. Instead of standing up for Hassan the way his friend had for him so many times, he fled. Amir tried to convince himself that he ran out of fear, but he knew that he felt Hassan to be his sacrificial lamb, the one to suffer for him so that he could live happily. In spite of himself, Amir thought, "He was just a Hazara, wasn't he?"