As I said, I could hazard a guess (or two) as to why: definite Borderline characteristics thanks to a medically recognised hormone imbalance ... but I suspect there was an underlying Antisocial disorder too.

Iit wasn't just the vengeful/petulant behaviour of someone overly upset, but calculatedly cruel (which tied in with other ... calmly cold ... behaviours, gives me reason to suspect the behaviour would've been, perhaps less overt but still the same, even without the hormone imbalance) - dunno ... I sensibly didn't hang around long enough to learn the hard way and make an informed diagnosis, so I can't (but my gut tells me she was a sociopath/psychopath and the hormonal imbalance simply made it impossible for her to hide it any more, because she was psychoemotionally out of control).