Dude may have had her financially strapped or threatened her with hurting her children if she left...
So she stuck it out to ensure their lives. She'd take the brunt of his anger 'cuz maybe she wasn't necessarily afraid of it, more that she could tolerate it and knew that her kids could not.
Kinna like when the father's a sexual child abuser and molests his 3 year old daughter till she's outta the house and the mother knows all about it and does nothing to stop it...
We will never know what goes on in the mind of the criminally insane...
This situation is sad