Hitch Hike to Hell(1977)

Howard is mild-mannered and slightly simple-minded, with a habit of picking up teenage hitchhikers while driving his delivery routes. Sometimes the girls admit to being runaways, and if they claim to hate their mothers it drives Howard into a violent frenzy; his sister ran away from home years ago and was never heard from again, causing his desperate, addled mother to tighten her hold on him. Howard never remembers raping his victims or strangling them with wire coat hangers, though his boss does notice missed deliveries and late arrivals.