I think she knew what she was doing but had cracked under the pressure
It does sound that way doesn't it.
it looks like it was premeditated so that instantly means it's murder in common law.
She had the car window open enough for her to get herself out, so she must have planned that part of it.
She was taking a huge risk though, what if she had got trapped trying to get out? Anything could have gone wrong, so I think she must have been quite desperate to take that course of action.
I'm not saying that the police could have done much, but she should have reported his behaviour towards her, if not stalking, it's definitely harassment.
Very very sad for all concerned :-(