mkcapen1 I love your response regarding Amir's character response, but I also want to add some literary context.
While Amir would absolutely feel this way, we must also consider Hosseini's purpose in having such an unlikely thing happen; in other words, WHY must it be Assef?
The Kite Runner can be considered an epic tale. It investigates heroic feats that better the protagonist and most characters are one-dimensional in the sense that they can easily be distinguished as good or evil. Therefore, while Amir's response is undoubtedly one of shock, fear, and horror, because he understand how despicable this man is, deep down he understand this encounter is necessary for the redemption that he seeks.
I don't know at what point I started laughing, but I did. It hurt to laugh, hurt my jaws, my ribs, my throat. But I was laughing and laughing. And the harder I laughed, the harder he kicked me, punched me, scratched me.
"WHAT'S SO FUNNY?" Assef kept roaring with each blow. His spittle landed in my eye. Sohrab screamed.
"WHAT'S SO FUNNY?" Assef bellowed. Another rib snapped, this time left lower. What was so funny was that, for the first time since the winter of 1975, I felt at peace. I laughed because I saw that, in some hidden nook in a corner of my mind, I'd even been looking forward to this.
In the book The Kite Runner Amir grew up in a neighborhood in Afghanistan that was bullied by Assef. Assef would taunt him and mock Hassan. On one incident Assef had cornered Amir and tried to fight him. The fight was stopped by Hassan who pulled out his slingshot.
The next situation was more serious. Hassan had run to get Amr's kite as his kite runner. Assef caught Hassan and in front of his friends he raped Hassan. Amir had seen it but hid his secret inside himself. It had hauted him his whole life.
Assef was invited to Amir's birthday party by Amir's father. Assef gave Amir a present. It was a book about Hitler. Assef respected Hitler and thought he was right about race purification. Amir withdrew from the party and threw the book away.
When Amir realizes that the Taliban man is Assef his stomach must have wanted to vomit, and he probably felt terror. As a child Assef was evil and to have him sitting before Amir in control of the situation must have been frightening for Amir. He was finally having to face his past and deal with the devil.