Bad Ronald (1974)

Ronald Wilby is a maladjusted teen who accidentally kills a young girl whose sister spurned his romantic advances. Ronald’s doting mother decides to protect her son by creating a concealed room in their home in which he will live. When the mother dies suddenly, Ronald stays hidden in the room, even after a new family moves into the home. Ronald uses his secret spaces to spy on the family, eventually taking one of the young girls hostage, with the hope of making her part of his secret world.