Four Flies on Grey Velvet (1971) on Netflix

The drummer of a rock and roll band Roberto Tobias sees a man wearing sunglasses stalking him everywhere. He follows the man to a derelict opera house and when he confronts the stranger, he pulls a knife. However Roberto accidentally stabs him in self-defense and the man falls from the stage to the floor. Out of the blue, a spotlight is turned on and Roberto is photographed in the crime scene by a masked person on a theater box. Roberto leaves the place and returns home. On the next morning, he reads the newspaper the news about a stranger called Carlo Marosi that was murdered. During the night, Roberto and his wife Nina welcome friends and Roberto finds a photo of the previous night entwined with his vinyls. He tells the truth to Nina and their maid Amelia overhears the story. While Robeto meets his friend Godfrey to ask for help, Amelia schedules an encounter in the park with someone to blackmail with the photos she had found in the apartment; however Amelia is murdered in the park Meanwhile Nina's cousin Dalia arrives to stay with Nina. But soon Carlo Marosi appears eating at a restaurant and calling someone to meet him. What has happened in the opera house?