She is deeply unhappy with herself. All the bravado and faux beauty is to compensate for the fact that she knows she is a train wreck of a person.
When someone does something that could remotely, even slightly expose her imperfections she goes on the offensive, Offence isn't just her best form of defence, its her ONLY form of defence, because despite her bigging herself up as this wonderful person who virtually sees herself as flawless royalty, the truth is she actually sees herself as indefensible.
I think this is true for a lot of bullies.