Horror movie from 80's?the town turns out to all be devil worshipers?

all i can remember is a lady in a large backyard next to an in ground swimming pool,she was next to the swimming pool bathing nude in blood maybe inside a small kids pool or something.there might have been a guy in the house watching her up stairs.i have been wondering what this movie is for a long time,please search your brains horror fans.i know there is someone out there who knows.