Halston: Don't worry, Drogan. I'll kill your cat for you.
Drogan: Good. Kill it, bury it, and bring me its tail so I can throw it in the fire and watch it burn.
Drogan: We were a dull collection of rich, old, unhappy people.
Cabbie: Hey, man, what's so fuckin' funny?
Bellingham: Oh, I was just thinkin' of this guy I know. Couldn't distinguish a third dynasty sacred scroll from a piece of post-Alexandrian pictogram porn.
Carola: You promised you'd never tell.
Answer: While it's not explicitly said in the film, it seems Preston breaking his vow turned her back into a gargoyle. She didn't have a choice and since he swore on his life never to say anything, she had to kill him. Plus, the stories in the book don't have happy endings.
Bishop73