Because telling a kid he has demons inside him and freaking him out is really going to help.
Images of death, violence and all kind of evil are prevalent. Considering what he has been exposed to, it is understandable that he learned that from what he has seen in images, possibly TV, video games, movies... This has happened during a developmental phase when his imagination has emerged. He is just copying what he has seen and heard, and not displaying something revealed to him or influenced by a demon that is possessing him.
This is a child who is suffering because of family instability, exposure to bad things at an early age, and possibly psychiatric health problems. Labeling him as demon possessed is not going to help his self esteem or encourage him to "be good." What this child needs is a mental health evaluation, treatment, behavior modification, unconditional love, support, and a stable environment.