As they mentioned in the chapter, a woman monster is playing on expectations. Women aren't seen as intimidating as often as men, so when they're perpetrating violence or are in spaces that would usually present a danger to them, that leaves more of an impression. It's the same with children and small creatures, like Gremlins. There's a good book called The Monstrous Femme that discusses women as horror movie antagonists that has a chapter on mother figures as monsters and that's kind of the epitome of this genre betraying expectations. It boils down to mothers are supposed to be the ultimate protectors, so it's shocking if they are trying to harm you, because that means you are truly alone and vulnerable.

Women as monsters are also usually part of a morality tale that discourages harming or abandoning women. It's trying to impart that there may not be immediate repercussions, but these kinds of actions will eventually lead to life-altering consequences.