The movie is a murder mystery that takes place at a theater where a magic show is performed and many of the lady assistants are killed.
This is the right type of movie, but this was made in the 70’s and in America. I am thinking it may have inspired the making of the “Wizard of Gore” movie. Now I am just guessing, but It might have been made in Britain. Someone posted a picture from the movie in our (now defunct) Dangerous Circus Acts group around 15 years ago. But thank you for responding
Maybe The Mad Magician - Wikipedia , but it is also an American film.
I strongly doubt this is the right film, but I will suggest it anyway. I half-watched a 1964 black and white British horror film, Devil Doll, on TCM a year or so ago. It was about a magician/hypnotist who murdered an assistant, and transferred the dead guy’s spirit into a large ventriloquist dummy. The magician controlled the doll and used it in his act, though occasionally the dead assistant regained control of his doll body and posed a temporary threat to the magician. I think the magician killed a second, female assistant later in the film (but not on stage), then put her replacement under his hypnotic control. It seems like you are looking for something much closer to Wizard of Gore, where the murders actually occur on stage during the act, and I wouldn’t describe Devil Doll as that kind of film. But it is British, from the 1960’s, and has a lot of scenes of his stage magic act, and the magician was behind a couple of murders of his assistants. So, there is a small chance this might be the film you are looking for.
though not a theatre, the film berserk set in a circus shows a scene where an assistant gets cut in half , if you have only seen a still this could be it.