Revealing mistake: In "Cat from Hell" the cat attacks two people; Carolyn and Haslton, by clawing onto their faces. When this happens, and the victims struggle, it is all too obvious that a fake, stiff cat is used instead. (00:41:50 - 00:53:40)
Revealing mistake: In "Cat from Hell" the cat crawls back out of Halston's mouth after the clock strikes twelve at Drogan's return. As it exits, the mouth and face collapses and is quite obviously made of rubber. (00:54:45)
Revealing mistake: In "Cat from Hell" Drogan has a heart attack in his wheelchair, due to the cat frightening him to death. But in the final shot of this story you can see Drogan blink, even though he is dead. (00:55:25)
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