Twisted Nerve (1968)

Martin Durnley (Hywel Bennett) is a troubled young man. With a mother who insists on treating him like a child, a stepfather who can't wait to see the back of him, and a brother with Down Syndrome shut away in an institution, is it any wonder he retreats into an alternate personality--that of six-year-old Georgie? It is Georgie who befriends Susan Harper (Hayley Mills), but friendship soon turns into obsession. When Susan begins to distance herself, something inside Georgie snaps and he embarks on a killing spree, with Susan as the next target.