In The Exorcist III, as the plot unravels, it is clear that the demon possessing Regan McNeil made the spirit of Gemini Killer to possess elderly folks from the hospital to commit those murders.
First the black boy, then that priest got decapitated in the confession booth, then Father Dryer in that hospital bed.
What is the motivation behind those murders? Was it revenge because of what happened in the first movie, the demon wanted to exact revenge to priests in general? If so then why the boy?
The movie doesn't go into it as much as the novel (Legion) does, but the Gemini Killer's motive is to shame his father, whom he hated. His victims all have a name beginning with 'K', as his father did: Thomas Kintry, Father Joseph Kevin Dyer, Father Kanavan, Nurse Amy Keating, Mary Kinderman (attempted). Not to mention he's possessing Father Damien Karras!
That many of his victims are related to the church or people involved in the original Exorcism is the influence of the demon. Kintry was (if I recall correctly) an altar boy at his church, the priests are obvious (and Dyer was a close personal friend of Karras), the Nurse was a devout Christian, Mary is Karass' friends daughter etc.
Quote: "The Gemini's spiritual "master", who had possessed the girl Regan MacNeil, was furious at being pushed out of the child's body and is exacting its revenge by putting the soul of the Gemini Killer into the body of Father Karras. Each evening, the soul of the Gemini leaves the body of Karras and possesses the elderly people with senile dementia elsewhere in the hospital and uses them to commit the murders."
Source Wiki. Writer's subplots.
