Ash Redfern: Dragon Slaying
Daughters of Darkness ends with Ash leaving Briar Creek for a year in order to "make up" for some of the things he has done in the past.
When she saw into his mind, Mary-Lynnette sensed that Ash was ashamed of the things that he had done in the past, but she felt that "shame didn't change the acts themselves." This is true, but the shame changes Ash. He is not the person he was when he committed those crimes, whatever they were, and he is truly repentant.
Will he be able to make up for those acts? No. Despite Ash's resolve, no amount of good deeds will change his past. His past will always be his past, it will not change. He, however, has and will continue to change. What he and Mary-Lynnette need to learn to do, is to accept his past and make sure he does not make the same mistakes again.
Did Mary-Lynnette realise that Ash would never be able to change his past? Perhaps, on some level, she sent him away on his quest to provide an 'out clause' for their relationship. This way, if she doesn't like being with him, she can always fall back on the excuse that he hasn't made up for his past actions yet.