It is movie before 1990, definitely.
It is a crime or horror movie about the investigation of ritual murders committed by a woman on men while they were alone and about to have sex. The corpses were always found in bed and naked, so the suspicion was that it was the woman who murdered them. The murder was carried out with a special knife and she would dress for the deed in such a huge stylish fear-inspiring gown, the very sight of which sent a shudder through her. In normal life she was an ordinary nice woman.
At the end of the film perhaps the policeman or the main character who investigated these murders eliminated her from the circle of suspects due to a coincidence and some mistake, went with her to do these things and was already lying in bed, I do not remember if he somehow drugged them, she went to the bathroom supposedly to wash up, and he waited asleep or she put him to sleep and and she showed up in this dress with a knife and at the last moment ( as it happens in movies) did someone accidentally after time discovered this mistake and they had to go quickly to save him and that she was caught in the act.