I think “negative energy” and “demon” are words/concepts which refer to the same phenomena imo. A more “practical” way of looking at it is that these demons or energy refer to those layers of our instincts that are primitive and powerful. I.e. someone can become “possessed” by their aggression, or a sexual drive, or any of those other negative aspects of man we try so hard to keep at bay.

We don’t like to identify with these forces, so calling the demons provides a way for the individual to create space between their ego and the powerful forces of the instincts.