In my opinion, demons don't exist as a spirit or a physical being that were created by a supposedly benevolent god to torture, harass, abuse, and steal the souls of humans. I think the authors of the portions of the bible that speak about demons were either speaking metaphorically, or trying to explain things they could not understand fully, or trying to create something that would scare the people into wanting to believe and not stray from the faith.
If they were speaking metaphorically, I would assume they were talking about humanity's own destructive nature, whether it be self-destruction or not. We have all heard alcoholism and other addictions called demons, and I think it is similar to what the authors may have been referring to. Temptation, deceit, addiction, and general mischief is in human's nature, but can all be attributed to biblical demons. I think it is those traits that are the actual demons, and there isn't a puppet master with cloven hooves and a pointy tail behind the evil that humans can portray.
If they were trying to explain things that they didn't fully understand, I would be inclined to think that it was obvious that those same traits of humanity were not understood, psychology wasn't a big field in those days, so they attributed it to some supernatural entity. They attributed almost everything they couldn't explain to something supernatural, why would this be any different?
The ones who were using demons to oppress and scare people into believing probably came later, and enforced the idea of supernatural demons and made it sound like these things preyed on weak Christians or people of other faiths, therefore causing people to be too fearful to educate themselves on other beliefs, or to rebel against the leaders of the church.
Demons are scary, so much so that people are fearful of them to this day. JustNormal has had some kind of 'demonic' haunting, and although I don't believe in demons the same way she does, something scary and powerful enough happened to her to make her honestly believe they were biblical demons. I can't explain what happened to her, as I was not there and probably do not have enough proper training. But I think that had she not already known and believed in demons, she would have attributed the unexplainable things happening to her to something else (i.e. aliens, ghosts, etc.) Demons are real to her because that's how scared people still are of them.
(JN I just want to throw in here that I don't want to debate with you, I know something horrible happened to you, and I respect your beliefs, and I am not trying to change your mind. I am just glad that you don't have the problem anymore, whatever it was, and I was just showing my opinion on your particular situation. I of course, might not be right.

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