The Alfred Hitchcock Hour (1962) : 3x29

Johnny Kendall is a trigger-happy cop who is fired from the police and is forced to take a job checking vacant summer homes during the off-season. His girlfriend Sandy gets a job as a waitress to try to help them make ends meet. At work, Johnny discovers that the previous deputy was fired from the job for having an affair with an unidentified woman in one of the vacant summer homes. Later, when Johnny sees the ex-deputy hanging around Sandy he becomes jealous and paranoid. One night, when he believes the two are together, he takes his gun and begins to look for them. He heads for the empty summer houses and finds one broken into. Inside he hears the former deputy and a woman. He shoots them both in the dark and kills them. He turns on the lights and discovers that the woman was not Sandy, but the sheriff's wife.