Rahim Khan tells Amir that Baba was Hassan's father. Amir is shocked and taken aback by the news. His level of anger is brought on by this sudden news. Another level of his frustration is in the fact that Hassan is gone and Amir is left powerless to do anything to impact the arc of their relationship. When he reads the letter, Amir is moved to a point that is undercut with the realization of his death. The news that Hassan was Amir's half- brother is powerful enough to cause intense questioning of his actions and his entire being in the world.
Amir's anger is also driven by the fact that his life to that had point had been built on a deception. It is the type of deception that causes Amir to have to reflect back on his life, searching for signs that might have tipped him off on the truth. For Amir, this anger resides in having been denied for so long a critical piece of news. It is in this way where Amir's anger is so strong and potent. It reflects a condition of being in which the past is no longer readily accepted and the present is a muddle. This becomes the result of the secret revealed and why Amir is so angry about what he learns.