I don't believe in it either. The demon stories were made up to scare people into doing whatever the priests said. JW's have had their share of BS demon stories too (got to love the scary pictures in the WT's and books). I remember being so scared as a kid that "demons would possess me" if I did anything to entertain them i.e., read Stephen King novels (I swear my parents said if I read his books it would please the devil), wear black nail polish (I kid you not), listen to heavy metal music (the lyrics were Satanic), etc. lol
However, I do believe, especially years ago before modern medicine, that people who were considered "demonized" or "possessed by the devil" were really people with brain disorders, skitzo's, etc. They obviously acted "different" from society (things like foaming at the mouth, acting crazy, etc.) made them appear demonized and they were brought to priests to be exercised of their demons. Crazy!