what did amir want to do while everyone was sleeping at kaka homayoun's house?

I think that you are referring to Chapter 8. Amir has been in a state of denial about the rape of Hassan. When all were spending the night at Kaka Homayoun's house, Amir confessed, more or less to himself, that "I watched Hassan get raped". The guilt if running away, while his Hassan struggled and pretending that it did not happen had been too much for Amir to endure.