What did Rahim Khan mean By "there's a way to be good again"? Why is it important for Amir to redeem himself? If so to whom and why?

Amir, in his childhood, behaved in ways that caused
grievous wrongs to Hassan and Hassan's father. In fact, it could be reasonably said that
his behavior played a part in their deaths.

While Amir is
generally arrogant and domineering to Hassan when they are children, the first really
striking wrong we see in the novel is when Amir hides in an alley while a group of
bullies sodomize Hassan.

Since the presence of Hassan in
the household is a constant reminder to Amir of his own wrongdoing, he manipulates
events so that Hassan is accused of a theft, and that causes Amir's father, Baba,  to
"exile" Hassan and Hassan's father, Ali,  from the
household.

Because of the subsequent upheaval in
Afghanistan, Amir and Baba are forced to flee the country. Had Hassan and Ali remained
in the household, there is a good chance they would have left for America with Amir and
Baba.  Both must remain and both die.

A chance to be good
again is a chance for redemption, which is a prevailing theme of the novel.  While Amir
and Ali have died, Amir has left a son behind, Sorhab, who will have no chance for a
good life unless Amir can rescue him.  Such a rescue is a way for Amir to overcome his
own cowardice, atone for his sins against the family, to redeem himself in his own eyes,
and to save one child from a terrible life or death.

You
will notice that the book opens with the idea of redemption and closes with the
redemption, telling the reader what a powerful and important idea this is in the
story.