L'étrangleur / The Strangler (1970)

Unhappy women are being murdered by Emile (Jacques Perrin), a psychotic young man suffering from the delusion that his acts are mercy killings. The detective (Julien Guiomar) assigned to track down the killer resorts to seriously unorthodox and even unethical methods to get his man. In one instance, he impersonates a psychologist on a TV show he and Emile appear on together and attempts to provoke Emile into revealing himself.

IMDB