A girl has a black cat who she thinks is turning her evil

[Kacy]

I is a book for youngish children i think and involves a young girl who takes in a stray cat around halloween, who is very ugly and black, and she believes the cat is speaking to her and telling her to do evil things or kill herself? I think it was used as a symbol for her deteriorating mental state? I was in early middle school when i read this but stopped because it creeped me out.

[Rainbowheart]

The Witches of Worm?

[slauderdale]

This definitely sounds like The Witches of Worm. It's been published with a number of different covers, but I always remembered the yellow one from the 70s.

[Rainbowheart]

Still out there, Kacy?