To answer the question about why women can be so mean to each other, I believe that many of the most extreme instances of relational abuse can be traced to character defects, such as malignant narcissism and borderline personality disorder. My posts here tend to focus on narcissism, since, of the two conditions, this is the most dangerous one. Narcissists, in general, are not as impulsive as someone who psychologists would consider a "borderline." A narc can often function very well in society, and appear perfectly normal. Because she can come across as being very competent, people tend to trust her and believe what she says.
However, someone with the classic symptoms of borderline personality disorder may appear a little odd or eccentric. Unlike a narcissist, she can have a empathy and go out of her way to help you, until she turns on you. Borderlines will inevitably flip 100 degrees, and can go from loving you to hating you in an instant. When you're the enemy, they can become very aggressive. During this time, they will go on a full-scale, all-out assault to turn other people against you.
Although the two conditions share a lot of common traits, narcissism, especially the malignant kind, is the most problematic. That's because of all the stealth attacks. It's very clear when a borderline doesn't like you. She glares at you and she's likely to insult you to your face. However, a malignant narcissist may be seething with hate, but still smile sweetly at you while she's plotting your destruction.
Pixabay image top by Nemo