What is the significance of Amir’s dream? Why does he dream that he is Hassan’s murderer?

Amir slept restlessly, dreaming about Hassan's death. He imagined that he himself was the Talib executing Hassan. Amir has been carrying all sorts of guilt ridden feelings for Hassan. From his rape to the fact that Amir left for a nice life USA, Amir feels that he is responsible for Hassan's murder.