I would be utterly destroyed if my abusive spouse (If I had one, which I don't; been there done that, no thanks and never again.) killed my child, more so if my child was trying to protect me at the time. I don't think I could ever come back from that.
I'm the type that if you hurt me, you probably won't get much of a reaction, but hurt someone else, especially someone I care about and you won't believe the difference in my reaction. Hell, I could hardly believe the difference in my reaction. (Not a scenario like this, but similar concept of my being abused leading to someone other than myself being hurt.)
So, until otherwise, I'm going to give her the benefit of the doubt and assume that she just never thought he'd hurt anyone but her. Her daughter is 28 and thus not technically trapped in the relationship with her mom, so convincing herself that only she was in danger was probably easier than it would be with small children.