If the abuse is based in the addictions it usually changes for the better when the addict gets clean and sober. Ongoing couples therapy, and a 12 step recovery program for both partners is still advised. It is important to recognize that abusive relationships are horribly painful for both parties. These relationships are usually highly reactive and drama driven, therefore mutually abusive on some level. Abusers don't make a choice to be abusive, they are generally doing the best they can given the emotional and psychological issues they face.
Abusers are often survivors of abuse themselves.
Many of the attributes of abusers are documented trauma based adaptations to childhood emotional, physical and sexual abuse. Abusers act out of deep seated shame and feelings of inadaquacy. They seek to pull thier partner down to make themselves feel better. Abuse is a family dysfunction that repeats through generations. Just as addictions pass down through generations, abusers often leave their families for a family of choice - then repeat the abusive cycle from the other side.