The Night Digger (1971)
Maura Prince works part-time as a speech therapist and the rest of her time taking care of her blind, invalid mother, Edith. Billy Jarvis arrives, claiming he was sent there in place of a neighbor's nephew who was to live with the Princes and work in their garden. He ingratiates himself with Edith, who gives him Maura's bedroom and claims he must be a long-lost relative. Despite Maura's worries, Billy turns out to be a helpful assistant.
While attending church with Edith, Billy notices pretty, young nursery school teacher Mary Wingate. That night, while having psychotic flashbacks of being raped by women and taunted for impotency, Billy goes to her home and murders her, then buries her body at a road construction site. The Princes' neighbors discuss the murder, which is the seventh in a series of killings of women. A nurse visits Edith and warns that Edith's heart is very bad. Edith, impatient when Billy doesn't answer her calls, tries to climb the stairs to his room and has a heart attack. Maura rushes her to the hospital. The nurse, returning to check on Edith, finds Billy there alone. Gripped by another bout of psychosis, he follows the nurse home and murders her as well.