Sula ends with a message about how an inability or unwillingness to grant compassion and forgiveness can have a lasting effect on a person. Compassion and forgiveness aren't always easy to muster, and we certainly understand why some characters are unable to forgive for a long, long time. But we also learn that an inability to forgive can lead to regret.
We should forgive Sula for her actions since she is a product of her environment and her childhood.
There are some things that are unforgivable between friends. Sula ruined Nel's life and doesn't show any remorse, so she doesn't deserve Nel's compassion.