The 2007 film Atonement, starring Keira Knightley, is the doleful story of a woman attempting to right a wrong she committed as a thirteen year old in 1935 — a wrong that ruins the lives of her older sister and her lover.
Atonement means “satisfaction or reparation for a wrong or injury; amends.”
A person who atones for a wrong is contrite.
A person who does penance could also be said to be penitent (antonym: impenitent). The word penitent comes from the Latin word for punishment, which also gives us penal and penitentiary. Also relatedly, if you did something in a non-punishing way, you could describe it with a very interesting adverb:
While it’s true she was still angry at him, she wanted to emphasize that she canceled his plane ticket only for financial reasons — that is, she did it wholly unpenally.