As pointed out earlier Paiviaso these 'men' do not start out this way.
Had someone told me 8 years ago that I would be living the way I was now, with a man like DH, I would, quite literally, have wet myself laughing. Or slapped them.
When I met him he was kind, helpful, thoughtful, great with dd1, excellent with my dog, made a massive effort to like my cats, despite hating cats.
He was the kind of man who come and alleviate my sisters from babysitting duties early so he could clean up the mess they'd leave me when I was working nights and make sure I had lunch to come home to. He'd then take dd1 out to the park or for a walk so I could sleep and come home and a cook me a meal and run me a bath for when I woke.
This is the same man, who now, is incapable of reading his own children, whom he lives with, a bedtime story, or even ensuring I have a clean pots and pans to make my own supper with when I finish work at midnight, him having spent the night on the sofa with his feet up, hugging the dog he wanted to keep, who he has not bothered to train and then moaned at me when she won't stop jumping up at him 
He didn't change overnight. It happened so slowly that I didn't spot it until I was already exhausted by it. He lunged from one crisis to the next, making me feel unable to leave. How could I leave when he was depressed? Bereaved? Had just lost his job? Was fearing he had cancer? Is at risk of losing another job? etc. etc. etc.
He's also clever enough to do just enough to leave me looking like an ungrateful madam when I moan or to go and purchase something big and fancy for me when I talk about leaving. Usually on credit. Then what? I can just walk out and he can tell people how I have left him lumbered with debt after he worked so hard to provide for us all. Or he will book a holiday and then get the children all excited about going away as a family.
He also gets the kids on his side picking out and emphasising things I haven't done or don't do enough and then going on at them about it until they repeat it to others i.e. "Mammy said she would bake with us and then didn't. Mammy never does anything with us, she always promises to and then never does. Daddy took us to the park, yesterday" Yup, he did. And left all the washing and cleaning for Mammy's day off so mammy has no time. I could just leave it, but then we'd be living in squalor and that would be my fault too.
And things will change. Things will always change when the next crisis ends. Things were going to be better when we moved. Things were going to better once he had recovered from losing his nephew. Things were going to be better once he left his job he hated. Things were going to be better when he found a new job. Things are now going to be better when his job is no longer at risk and he has started his counselling and found out what is wrong with him. Or he will be dead. His illness might possibly be terminal now 
here's hoping, eh?
Plus we haven't argued for months, things are getting better, he is trying. Um, no. I've just given up, that's what's changed, you're no longer verbally abusive because I have given up challenging you, so there is no need to be.
There is 4k sitting in an account I have access to. I could, theoretically, just go now. Go and put a deposit down on a house right now and be gone before he gets home from work. But how can I do that when the children are looking forward to their holiday in a few weeks and DH is having blood tests because has something seriously wrong, not to mention the fact that he now needs counselling to help with his anxiety and depression. Who would get the sympathy and support? And I'm not strong enough to do it alone. I'm shattered.
To the outside world he is a good man who works hard to provide for us all, despite his medical and MH issues. I expect too much of him.