Do you know this short story about a body-snatching witch with a demonic bicycle?
She gets a high school kid to fall in love with a girl, and then somehow manipulates things so that the school bully gets involved. Then they're supposed to fight over the girl and when one of them gets killed, she uses that (negative energy, I suppose) to steal the girl's body and live forever.
It was in a book of short stories I bought some time last year, but the book seems to have disappeared. Does it sound familiar?