A Lizard In a Woman's Skin/Una Lucertola Con La Pelle Di Donna (1971)

In London, Carol Hammond lives in a fancy building with her husband Frank and her stepdaughter Joan. Carol is the beloved daughter of wealthy, prominent lawyer and politician Edmond Brighton; Frank is his partner in his office and has a love affair with Deborah. Carol's next-door neighbor Julia Durer is a depraved woman who promotes parties with drugs and orgies. Carol has psychoanalysis sessions with Dr. Kerr and is intrigued with a nightmare where she stabs Julia to death three times while a couple watches. When Julia is found dead in her apartment, efficient Inspector Corvin and his partner Sgt. Brandon are assigned to investigate.

IMDb: https://www.imdb.com/title/tt0067361