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Of course Amir deserves to be happy. Amir is at heart a kind intellectual who has suffered more than many of us can know. As for being punished by his sins, Amir's only sin was being scared of a sociopath Assef. I don't think the universe unfolds in such prescribed way.
Amir deserves to be happy because he was living life where Baba lied to him for all those years. Yes he did watch Hassan get raped, but he was just scared of Assef. He was not loyal to Hassan because Hassan saved him eariler in the story but Amir was a very kind character with only a couple flaws that made him a dark character in some parts of the book