Homicidal represents producer/director William Castle's slant on Hitchcock's Psycho. The film concerns a young woman named Miriam Webster (Patricia Breslin) who seemingly has everything a girl could want - including a successful flower shop business, and a handsome beau, Karl (Glenn Corbett), who works as a pharmacist.
Events take a turn for the worse, however, when Miriam's half-brother, Warren, returns from Europe - with a rather unpleasant friend in-tow: a blonde named Emily (Jean Arless).
Emily promptly sets about destroying Miriam's life: the newcomer attempts to wheedle Karl away from Miriam, then rips the flower shop to pieces, then ultimately reveals a little taste for knife-wielding that directly threatens Miriam's safety.
Like The Tingler and other Castle outings, this one originally featured a gimmick, preserved in the video release: a fright-break just prior to the climax, which allowed terrified audience members approximately 45 seconds to get out of their seats and leave the theater - to avoid the prospect of being frightened to death.
One look at Jean Arless's credit in the cast listing betrays the final twist in this one, directly (and unapologetically) lifted by Castle from Psycho.