We can fault a lot of our fixation on demons - and what eventually became our little picture of Hell - to a cascade of events that started with Planescape: Torment. Fixation on the idea of a game that could move people emotionally, tied to one that spun around demons to begin with.
Intrinsic good and evil was where things got frustrating, to us. And so a younger Vespisys started unpacking the idea.
As conveyed, in so much media, to our understanding then and there: An angel is a thing so much indistinguishably body, soul, and the concept of "good" in an inseparable mass. Likewise, demons with "evil."
Obviously the world is not clearly that clean and distinct. So we took out the good and evil part, and latched onto the idea of beings who were, in some part, a concept as much as body and soul.
Exposure to a part of existence as a thing of sustenance, avoidance of it as a thing of starvation. To be tied to joy is to be sustained by the presence of joy, without taking away from it. And so on.
We've got so, so many younger-times journals filled with these little stories of demons and angels that really aren't that distinguishable from each other, all wearing their little ideas around their hearts, playing in their world.
I know this probably isn't exactly novel, as far as concepts go for parts of a world. But hey, it's part of what got us here. (Enough that it's even part of the otherkin-shapes of some people in the system here.)
Transition just made it a bit more personal, and was the catalyst towards tying it to ourselves, then identities and lives at large. The weird little katabasis of fleeing the country, the whole death-and-rebirth of the self with our life falling apart and getting rebuilt.
...just personal rambles, I suppose. We like building these worlds. It's nice to feel them becoming a bit queerer and a bit kinder in time.