I get it too. I remember a scene in Breaking Bad where a man watches a drug addict woman choke on her vomit in her sleep and die. And I remember thinking that if ExH had been choking on his vomit, I would have let him and done nothing and then pretended I slept through it.
When I realised that I would have preferred to watch him die than live another day with him, I left. Pregnant and with a 2&3 year old. I'm the happiest I have ever ever been now.