I kind of wonder if the mom could have possibly been abused herself earlier in her life? Not by this man of course but someone else? In a way it almost sounds like a PTSS related response. If that was the case a lot of anger could come out from the flash back especially if it had been repressed and suddenly brought back to conscious or subconscious memory. In some cases the event lays dormant for years until something triggers the memory. It can be something as simple as the way someone says something to them.
Consider women in general.
Most murders are committed by men.
Women are conditioned to sublimate anger and avoid violence.
It's in our societal nature to be non-violent and non-aggressive, really.
It's highly unusual to hear of a woman just up gunning somebody down.
Even crimes of passion that end in murder are more likely to be done by men.
[yes, I know that sounds sexist but I don't mean it to. men hunters; women nurturers]...;)
The most common female response is to gather up the child and flee from danger.
This woman acted out of character for her sex, historically speaking.
That alone makes me wonder if there's a huge backstory we're not hearing.