what was the monster that grabbed hassan by the ankles and dragged him to the bottom of the lake?

It is important to consider the full quote first,

“There was a monster in the lake. It had grabbed Hassan by the ankles, dragged him to the murky water. I was that monster.”

This particular part of the story has a few important parts to it. The first is that this is one instance where Amir is remembering something someone has said to him. I think that this is style, and it emphasizes the internal thoughts of Amir. He cannot escape the horrible thing he has done, and it effects how he views himself. It is a way of intensifying his guilt. Also, the entire metaphor is used to represent the act of Amir running away from Hassan’s rape. He drug Hassan to the bottom because he had a chance to save hims, but his cowardness kept him from helping. Also, the bottom is Hassan losing innocence, friendship, and feeling unworthy and dirty about himself. He does not respect himself.