Some ghosts could actually be demons.
I grew up in a house that was haunted by something supernatural. Had a lot of weird and freaky stuff happen. When we moved out, we had a guy that was helping us move who was very tuned in to the spiritual realm and had seen demons before. He refused at first to go towards the back of the house because he immediately sensed there was something evil there. The back end of the house was where most of the freaky stuff occurred.
I don't believe there is any evidence to suggest demons exist. Naturally we have been told by fundies that demons inhabit our house! Well if that is the case they are welcome!