Most cops are fine - I know many, and they would pretty much never use force except as necessary on the job (well, OK there is the occasional off the record "warning" to someone who's messing with their family/close friends). I'm not sure that domestic violence is more prevalent in their marriages than in the general populace, but it's even harder for the abused spouse to leave when the abuser is part of the force that is supposed to protect us.
It doesn't help that it's such a macho job that they generally can't talk about how stressed they are from the job to their family. It's proven many times over that this kind of stress is a primary factor in cases of spousal or child abuse. Police often internalize the stress, which makes it pretty much impossible for there to be a family support network to help take that load off of them. Add in alcohol (another primary factor), and a family history of abuse (yet another primary factor), and you get a really really bad mix, especially since that person is trained in the use of force and is generally armed to the teeth. I think that abusers also tend to find each other and cover up for each other, so add that in and you have additional armed people enabling the abuse. Yikes.
The key to stopping it is to change police culture. It's touch, but I believe that in the next couple of generations (yes, this will be seriously long-term), this will happen. It helps to have more women on the force, and it will help if the military follows through and sets an example by destigmatizing mental health. Since lots of cops are ex-military, and police departments in general are paramilitary in design, it should rub off eventually.
//OK, rant over. Seriously, I hope that someday someone is able to fit the pieces together of these cases and bring the perpetrators to justice.
