This serves a dual purpose for the female bully. First, she gets to watch her target suffer, since what she's done is very cruel. (Malignant narcissists often derive pleasure from other people's pain.) With everyone questioning the target's actions and motives, the narcissist can now count on them not to offer any resistance when she launches a full-scale attack, designed to destroy the target.
Relationships are a game to a malignant narcissist. She wants to win at all costs. She views people as objects to be moved around in a game of emotional chess. A morally disordered person will spent an inordinate amount of time studying her moves, with a patience that would make a world-class chess player look impulsive.
Psychologists call this type of behavior "relational aggression." As it become more apparent that not all narcopaths and sociopaths are men, as was previously assumed, it's now known that female predators abuse their victims on a more emotional level, by marginalizing them and excluding them.
Although, if you're a target, it may seem as if the narcissist is flying high, this is only for a season. Her crash is coming. Narcopaths always end up losing, as their lies and deception eventually convict them.