Mine died a slow death, but the first time I actually considered divorce was when we got a kitten and H was abusive to him (holding to it and playing at blocking his breathing to the point of it peeing on him - served him right). I actually told him I'd leave him if he continued to stress the cat like that.
It was a real eye opener, because he was just being cruel to the poor thing.
At some point I forgot our wedding anniversary - because I didn't feel like celebrating it, really, although it was mostly unconscious. And there's always the hope that things can get better, and why rock the boat and so on.
Finally, when he threatened to leave, I told him that he could go - he didn't. And when he asked if I loved him, I said I wasn't sure.
Weeks after that he was physically violent with me and even though I gave his a 2nd chance (told him it was over if he did it again), I knew it would happen again and was just a matter of time.
It did, only a month later. I told him to leave and called the police when he refused.
Stupidly, I let him in again, but not reconciled. Because he had nowhere to go.
And said that I would only take him back if he was normal for a whole month - he had refused counselling.
Next day he was still a twat, and the next day he had a few drinks and threatened to kill himself and both me and our DS.
Within an hour I was out of the door with DS, to the police. And we never got back.
Through all of this, I was actually in a better position when I finally left him. I was closer to family and as things are he is pretty much out of our lives.
The point is, I think you realise it's over often a long time before you actually accept it and then a long time before you take action.