Twisted Nerve (1968)

A homicidal sociopath (Hywel Bennett), the product of an over doting mother (Phyllis Calvert), has a brother with Down's syndrome (called mongolism in the film). Attracted to a young girl (Hayley Mills), Bennett pretends to be a mentally challenged young boy and worms his way into the boarding house run by Mills' mother (Billie Whitelaw), herself a lonely widow with strong sexual desires which prove to be her undoing. Directed by Roy Boulting, this is an unsavory psychological thriller with a dubious medical premise. The film opens with a disclaimer that there is no proven link between "mongolism" and homicidal behavior. However, the film itself, none too subtly, seems to suggest that it does. It leaves a bad taste in the mouth but if you can overcome that, it's a decent enough suspense vehicle and the final moments are suitably intense. Alas, Bennett isn't very good. His simpleton act couldn't fool a five year old yet everyone around him has no problem buying into it. Boulting's direction is solid. There's a lovely Bernard Herrmann score whose main theme was appropriated by Quentin Tarantino for KILL BILL VOL. 1. With Frank Finlay, Barry Foster and Timothy West.