Trivia: The third story, " A Lovers Vow" was based on the Japanese folklore of the yuki-onna. However, writer Malcom McDowell decided that the creature should be a gargoyle instead of a spirit.
Trivia: George A. Romero wrote the screenplay for the segment "Cat From Hell," so director John Harrison put two references to Romero's work into this film as a kind of payment. In "Lot 249" when Lee is lying on the couch and the TV is on, it is playing Dawn of the Dead. Later on in the film, in "Cat from Hell," Halston is shown watching the Vampire classic, Martin, directed by Romero. (00:18:30 - 00:52:30)
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