"...like they’d cornered some kind of wild animal that only Assef could tame."

Hosseini uses figurative language to compare Hassan with a wild animal. As Assef and his two friends get closer to Hassan, it was as if Hassan was a helpless animal. Hassan was not even able to escape because they cornered him. In addition, since Hassan was by himself, there was no way that he could protect himself. So, it was like he just gave in and there was nothing more to do.