The Grand Unification Theory of Supernatural Evil

It's been more than twenty years, but the last time I was formally trained in physics, the Holy Grail of physicists was the Grand Unification Theory that would link conceptually the strong and weak subatomic forces with the electromagnetic force and gravity. To be honest with you, I'm not that concerned with unifying forces that occur at a subatomic level, but I am interested in merging forces that seem redunant. (We still haven't proved that those forces are redundant, for one thing.)

In particular, the genesis of supernatural evil in D&D is… overdone. The Monster Manual is rife with creatures that are supposedly powered by, if not generated by, supernatural evil force. Add in the other monster books and quickly this gets out of hand. You've got devils, demons, daemons (or yugoloths), demodands, night hags, kythons, hordlings, rakshasas, and… oh, I dunno, literally scores more of individual monsters that don’t belong to a greater class per se.

You've also got Undead, which are powered by Negative Energy. Conceptually, negative energy is quite silly in my opinion, and really at the end of the day, its just another euphemism for evil.
Why does D&D have this? Is it an artifact of any cultural genesis of the creatures being represented, i.e., a folkloric or mythological basis for this? No, absolutely not. If anything, folkloric and mythic basis would argue for a Grand Unification of Supernatural Evil. Does it make sense from a cosmological scheme endemic to D&D? Not really. Kinda sorta, in that several of the varities of fiend are there to represent the nine point alignment system, which makes, fundamentally, three kinds of evil which are separate from each other. And Negative Energy is the counterpart of Positive Energy. So, there's a weak endemic situation that this explains, but not well… it fails to explain a lot of other supernatural evil creatures, for example, as well as being… again, conceptually kinda silly. I don't like it, in other words.

Rather, I'd like to turn to a more folkloric basis for supernatural evil for my games. Something that's more consistent, which allows for as much variety as you want, yet has the same underlying philosophy behind it. A Grand Unified Theory of Supernatural Evil, if you will.
From a fundamental metaphysical level, then, this post will describe my "ideal" cosmology a bit more. This borrows somewhat from Judeo-Christian and Islamic thought (as does "mainline D&D") but I'm not attempting to make any comment on that. My sole purpose is to create a scenario that facilitates the kinds of games I want to run. This is because it makes it easier and funner for me, and for no other reason.

So, I'm also a "hands-off" kinda guy when it comes to cosmologies. Did the universe just appear, complete with metaphysical context one day, or was it created as such by some divine agent? I don't even want to answer that question, and evidence points both ways. One piece of evidence that points for the latter interpretation is the existence of spirits, intelligences, or sentiences of "evil"---those whose purpose is to undo the universe, or those that live in it. Where did these spirits come from? What is their genesis? Even they don't know. It's possible they simply appear as forces of cosmic opposition, although others have theorized that they come from "outside" creation somehow. These sentiences and forces animate undead as well as take corporeal form as various fiends. They don't choose to do so, but the end result is the same. Undead that contain the spark of the person they were prior to becoming undead are actually a fusion of a living mortal soul and this evil spirit.

For the most part, prior to incarnating as a fiend, these spirits are unaware and largely insentient. A very few exceptions exist; spirits that remember multiple lives as different types of evil creatures, but none of these remember their genesis; their memory does not stretch all the way to the beginning.

Non-supernatural evil, of course, requires no explanation. People are just bad. Frequently.