I think there are two different kinds of domestic abuse being talked about here, and being confused with each other. One kind is, as you mention KM, where there clearly one abuser and one victim. In that case, the abuser is usually a controller, a predator, and will go from partner to partner repeating the cycle of abuse. But not all domestic violence cases are like that, at least not in my opinion. I'll use my stepparents as an example.

Eric and Cheryl were happily married for a few years. They didn't have any major problems. Eric was in the military and he was deployed several times. After his last deployment he was developing PTSD, though no one knew it at the time. While Eric was on stress leave, he started drinking heavily and yelling at Cheryl and us kids. He'd spend days shut up in his room by himself. Cheryl yelled at him back just as much, and they had these horrible fights. Cheryl often started it, because she was upset about him being depressed and withdrawn. He would tell her to leave him alone, he would try to get away from her, I remember seeing him restrain her once because she was hitting him in the face. He was trying really hard not to hit her, because he was a lot larger than her physically. These altercations often occurred when they were both drinking. The violence escalated. Eric did end up hurting Cheryl. But really, who were the victims? Me and my brother. Not Cheryl. I refuse to see Eric as the sole abuser in this, and Cheryl as a victim. Cheryl was not a victim. She chose to act how she did, and did provoke Eric. She also did not protect my brother and I. She is was just as abusive as Eric in my opinion.

They had many choices. Cheryl could have left, Eric wasn't stopping her nor threatening her. Truth be told, he was depressed and wanted to be left alone. They could have sent my brother and I away, they could have got counseling... they had lots of options and they chose not to use them. Together. They were both at fault. I guess my point is, not all cases of domestic abuse are black and white. Lots of these cases are really complicated, there's lots of grey areas.