Continuity mistake: In "Lot 249" Andy sets the ancient Egyptian scroll alight. As he sits there in front of Bellingham talking to him, Andy keeps alternating from holding the scroll at the middle to at the edge, between shots. (00:30:50)
Visible crew/equipment: In the first part of the wraparound story, the film crew is reflected in the SUV Betty is driving as she pulls out of the parking space.
Continuity mistake: In "Lover's Vow", as Preston evaluates the deep, nasty scars he has on his chest from his encounter with the gargoyle, he turns away from the mirror for Carola to tend the wounds. But, when Carola starts treating the wounds, the deep wounds are now shallow streaks of blood rather than deep scars.
Visible crew/equipment: In the scene where the hitman is chasing the cat around the pool table, you can see the shadow of a tennis racquet on the floor as if being followed as a distraction for the cat to follow.
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