Enrique Guevara - Cariño mío, ¿qué me has hecho? aka Darling, What Have You Done to Me? (1979)

Chilean-born director Enrique Guevara was one of the foremost exponents of the erotic cinema which blossomed in Spain in the late 70s after the death of Franco and abolishment of censorship. These films, classified 'S' for sex content, were often more varied and inventive, and heavier on plot, than their counterparts in countries with more developed traditions of erotic movie-making. Guevara's preferred actress was his sister, Raquel Evans, who is better known today for her appearances in the films of Jess Franco. Cariño mio tells the story of an Oedipally-minded young man who loses his mother in an accident and goes to live with an aunt. The latter so much resembles his mother that the boy becomes obsessed with her and as a first step to possession decides to separate her from her partner,
Spoiler:
whom he kills. When the time comes to reveal his evil intentions to the lady, he discovers her big secret: she is a transvestite. He humiliates her and tells her that he killed both his mother and her partner, and intends to kill her, too. But the aunt's dog is alerted by the rumpus and comes to her aid, emasculating the young thug.