What was the driving force that compelled Amir to transform into the personality that was hidden within?How can I explain that the "cycle of...

Most of the impetus behind Amir's later actions,
particularly his return to Afghanistan to try to "make things right," are driven by the
guilt which has riven him ever since the day that Hassan was raped after chasing down
the kite for Amir.  He stood by and watched while his most loyal friend was brutally
molested, and that inaction has haunted him throughout the rest of his life, no matter
how far or how long he got away from the incident and the place where it
happened.

Particularly once he finds out that Hassan was,
in fact, his half-brother, Amir is desperate to find out whether he might be able to
somehow rectify the wrong he committed or felt he did at the
time.

In terms of a redemption cycle, it may be his
rescuing of Hassan's son from a possible future of constant abuse that he feels he has
finally atoned for his inaction after he is able to bring Sohrab back to the US with
him.