Homicidal is a 1961 thriller film produced and directed by the self-proclaimed "King of Showmanship", William Castle. Written by Robb White, the film stars Glenn Corbett, Patricia Breslin, Eugenie Leontovich, Alan Bunce, Richard Rust, and Joan Marshall (billed as Jean Arless). It was released with a "fright break" that allowed patrons to receive a refund if they were too scared to stay for the climax of the film.
A mysterious woman named Emily (Arless) convinces a stranger she just met, to meet her at a local justice of the peace to get married hours after they met. But inside the chapel, Emily stops the ceremony in order to savagely murder the local justice of the peace and flees. She later gloats to a mute, invalid elderly woman named Helga (who works as a nurse for a wealthy family) of her deed.
The police investigate the crime and learn that the nurse was given the name of a local flower shop owner Miriam Webster (Bresslin), who has an alibi of the night of the murder. It is revealed that Miriam and her cousin Warren (Arless), who has recently returned from abroad after the death of last surviving parent, are heirs to Warren's father's estate. The two talk about how Warren's father was abusive to Warren growing up and the details of the will. Miriam stands to inherit the estate if Warren were to die before marrying, as Warren's father was a misogynist that went out of his way to make Warren his sole heir, so that only a male child of his could inherit. Emily also confesses to Warren of good news of her own, that she was engaged to be married to her boyfriend.
That evening, Emily breaks into Miriam's flower shop and wrecks the store. However she is seen by Miriam's boyfriend, who did not know that Miriam had left early that evening. When the two arrive at Warren's house the next day, to visit Helga and encounter Emily. Helga frantically tries to communicate with Miriam, who later finds out from Warren that Emily is his wife: he hired her to take care of Helga while the two were away and ultimately married her. Miriam later overhears the two talking in another room but does not see them, as Warren calls Emily to bed, when Miriam demands to see Warren, when her boyfriend learns of the murdered justice of the peace and that Emily resembles the suspect.
Emily ultimately goes visit Warren and Emily, having realized that Emily is a murderer only to find Helga decapitated. Attached by Emily, Miriam and Emily fight and Emily is defeated and her hair removed, revealing a wig as Warren is revealed.
Afterwords, the police talk to Miriam as the truth about Warren came out: Warren was really a girl, raised as a boy by Helga and the justice of the peace (who was once a doctor and had delivered Warren), and his mother. This was done mainly to avoid the murderous wrath of Warren's father, who wanted a boy and would have harmed the child. "Emily" was an alternate identity Warren had created overseas to be able to live as a woman away from those who knew him. When Warren's father died and he learned of the clause in the will would have denied him his inheritance if his secret was revealed, he resumed the alter ego of Emily, in order to kill and silence those who would know the truth about him.