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?