The Prowler [Le Rôdeur] 1951

Webb Garwood, a disgruntled cop, is called to investigate a voyeur by Susan Gilvray. Her husband works nights as an overnight radio personality. The cop falls in love with the young and attractive married woman. Obsessed, he woos her despite her initial reluctance. Garwood finds out about an insurance policy on the husband's life. He dreams up a scheme in which a phantom "prowler" would be a good scapegoat if Susan's husband should happen to die mysteriously. After becoming a prowler himself, Garwood commits the murder and makes it look like self-defense...