In what ways do you feel that Amir has truly become good again?

I think Amir was a work in progress. By the time he risked his life to travel back to Afghanistan and rescue Sohrab, Amir had begun to deal with his demons. Taking the massive beating from Assef was cathartic for Amir. I think the last scene with Amir teaching Sohrab to fly the kite means there is yet hope.