Dunno - I've certainly come across abusive women. It's less likely to be physical abuse, but psychological abuse can be very destructive. And many are alcoholics etc.
As to why people abuse others - well they're usually from abusive backgrounds themselves. Maybe violence is societally more acceptable in men, but I'll never believe it's innate to men, any more than it's innate to women. I think men just get more opportunity to be violent, and it's more accepted. In fact it's often expected and praised. That's down to societal structures.
If we start saying men are essentially violent, then in my view we're saying that biology is destiny, and I don't believe that, for one minute.
By the way, the reason people usually don't report stuff to police and social services, is that they fear ending up on the receiving end of abuse themselves. I speak from personal experience, and the person I got the abuse from was the female 'victim' not the male perp. He was violent, but he also backed off the minute he thought the police might become involved. Like you, this wasn't a situation I got involved in by choice, but by proximity - in my case geographical. And you then find that in a lot of cases social services are absolutely useless the police were better IME, but they've improved in leaps and bounds recently in terms of dealing with domestic violence.
For some people, abusive behaviour is normal. It's all they've ever known, and they don't understand other ways of relating. For someone who didn't have that experience as a child, who was brought up to have empathy, it is incomprehensible. For people from abusive backgrounds, the idea that you can live without abusing people is incomprehensible - it's the only way they know how to survive psychologically or relate to people.
My honest advice to you is that it sounds like you're suffering from a sort of post traumatic stress, as we all would do if someone we know died so horribly - it's a very dark situation you've been dragged into. Try to distract yourself with nice stuff, long walks, art galleries, whatever you enjoy, get some support for you.
My view FWIW - you need to actually distance yourself psychologically from this if you can, you can't help this woman now, and you will drive yourself into mental illness if you continue to try to understand it. As I said before, I'm speaking from personal experience.
It's the old 'put the oxygen mask over your own face first' advice and it's true. You can't help anyone while you're not strong and healthy yourself.