The Alfred Hitchcock Hour (1962) : 1x2

At a college campus, two young girls are killed while strolling through a nearby forest. Suspicion falls on a chemistry professor and an intense and brooding music professor named Edwin Volck. Harold, a psychology professor, convinces his fiancee Daphne to act as bait to help trap the killer. The trap succeeds and Volck is revealed as the killer. Harold, however, becomes worried. He believes that the killers bloodlust may have inspired other less obvious psychos on campus. Daphne realizes that Harold has been speculating about himself when he suddenly attacks her. She is rescued by the chemistry professor. The chemistry professor had been watching Harold's behavior since the beginning believing that he was a possible psychopath.