My theory is that almost everything in Supernatural derives from human stock. Angels. Demons. Gods. Monsters. These are all based on the human template. The human soul is, in essence, a battery, and these monsters and creatures know how to tap into this battery to fuel their abilities. Some creatures, such as angels, demons, and gods, can use more than one soul at a time, granting them more power, and the truly powerful entities have a storehouse of scores, hundreds, or even thousands of souls to draw upon. Monsters are a corruption of the soul, creating 'something else'. For example, the First Vampire, which was spawned by Eve, was a person whose soul was corrupted. It bites someone else and infects that person's soul, making a new vampire. The corruption of the soul corrupts something fundamental about that person, making that person become "evil". Lycanthropy follows the same rules, and any other 'infection' based supernatural.
This also explains the weak 'gods' in Supernatural. These are entities who don't have the scads of souls going to them that they once had. Because of this, they aren't as powerful as the angels and "God" are, because the latter entities are getting hundreds of thousands of souls each generation. Essentially, Heaven is 'flush' with souls to draw upon to fuel themselves. This also means, according to my theories, that God was essentially a human, or something connected to humanity, but was simply able to draw on more souls than anyone else, and thus has a top-notch position in the spiritual side of things. This would also explain why Lucifer could tear down a bunch of gods in one meeting. He's flush with the souls from hell, and the demons he's devoured, and thus has an edge on anyone else.
Now, does Supernatural actually explain this? Yes and no. There are a number of hints and side-explanations, as well as a lot of 'show don't tell'. Sam was told that demons were human at one point. Humans can be given supernatural powers via demon blood (and that opens a lot of doors, now doesn't it?) Demons gain strength through souls. Angels get strength through souls. Eve was able to turn anyone into a monster. Leviathans can possess human bodies. Angels and demons can absorb the souls of Purgatory to fuel themselves (just like human souls). These are all connected, and shows an underlying pattern which the viewers can draw their own conclusions from.
Now, the crux of this. Should a show / book pull the curtain back and show how this all works? I don't think so. Unless the main characters have the means to find out 'the truth', and knowing 'the truth' is crucial to telling the current story, I see no reason for having the audience / reader be informed on the behind-the-scenes mechanics for everything. Let people decide why things are happening the way they are -- or not. Does it detract from the setting? Maybe. I don't think it does, but others might disagree.
As an aside, how does the mythology in Supernatural mesh with the setting? After all, there are a number of creatures in Supernatural who don't seem to fit with what we have in our folk tales and legends. Well, my explanation is this: normal people will usually get only the briefest glimpse of a supernatural creature in the series. Most, in fact, don't get the chance to survive the encounter. So, those who survive / get that partial glimpse write down what they see, make some strange theories, or attribute some things to the creature that don't actually exist. There's a perfect, real-life example of this in the kirin. The kirin is the Chinese dragon-horse, and has all sorts of things attributed to it. The kirin was spawned off an encounter with an actual creature - the giraffe. Now, the giraffe is not a dragon, and not a horse, and has no astonishing array of powers. A lot of things are attributed to the kirin, and thus you have a creature with strange powers and abilities which are nothing like the source material.
I figure, in Supernatural, this works the same way. The common folklore isn't accurate, but there can be kernals of fact within the folklore. The majority of information and lore that people like Sam and Dean draw from are not common materials. They draw from Hunters, who have gathered lore through trial and error, and who have this information stored (and probably now shared on private databases, and password-protected internet sites and the like). Actually, this conceit probably shows up in RPGs a lot, too. In games where the supernatural exists and blends into humanity, those people who hunt them down and who gather information on them wouldn't necessarily release this information publically - this stuff would be shared, yes, but it would be locked down and passworded, and thus only the people in the know would know how to access this information. Or, if some greenie comes in, they might have to register and get vetted to get access, but it wouldn't be open and public to all, because you have no idea what might stumble upon the information and try to sabotage it.
Now, should all this be explained in a TV series or book series? Perhaps more so in a book, since you have the time to give it out bits and pieces at a time as necessary, but in TV land it is much less likely. It isn't really going to keep the attention of your typical viewer, they'll just see Sam or Dean hitting the internet for lore, and coming up with some truly esoteric stuff. Why they can might be an exercise for the reader, but shouldn't really be a focus for the series. Or, who knows? Perhaps some of the lore they have is actually in more common books - indicating that a little bit of 'fact' seeped into the 'fiction' of the setting.
Okay, now let's look at world-building. If you're making an 'Earth' setting (set in any given time period), with a supernatural element, how closely do you have to cleave to the existing, real life folklore? If you say 'X exists', and you want to give some sort of Unified Theory for why they do, something has to give. Do you have to explain how this impacts the world's view of the supernatural? What if the world overall doesn't know?
Let's take werewolves as an example. Common mythology is that werewolves are shape shifters, they're tied to the full moon, they get killed by silver, may or may not be allergic to aconite, and regenerate. Some stories say that they're people who wear wolf pelts, some say it is caused by an infectious bite, and others say it's a bad person who wasn't buried properly (along the same vein as the vampire, who the werewolf's legend sometimes mingles with).
So, if you toss most of this out the window, and say, for example, "A werewolf is someone who, when angered, becomes larger, stronger, more bestial, and gradually regenerates from any wound except decapitation. The bite of the werewolf may pass on this sickness," how does this affect mythology in your setting? If you're trying to keep close to 'Analog Earth', you'd probably say that the legends as we know them are still there, and what everyone knows. Of course, a lot of that mythology is now flat-out wrong - no wolves, no wolfsbane, no silver, no full moon. If a PC decides to go searching for lore, they'll probably slam into the folklore, and be in for a rude awakening when they encounter the werewolf. Some veteran hunters would know the truth, however, and if the PCs encounter them, cool. If not, they might find a dusty tome or something which may give them a hunt of what's going on, but these golden nuggets of truth are probably buried under metric tonnes of crap.
Is this bad worldbuilding?
Now comes the question - if you don't know all the details on a given supernatural race, what's wrong with taking what you do know, filtering out what you don't like, and ignoring the rest? Is that bad worldbuilding? What's the difference between my werewolf example, me knowing some history on the evolution of the werewolf, and someone who hasn't, but makes the same end product?
And what if you want a Unified Field Theory involving supernaturals? 'All supernaturals are essentially X'. I'm doing this with my game Trionfi. Every single supernatural creature in the world comes from one place. They're all one thing, just manifesting as different evolutions of that one thing. A goblin is a spirit who has taken on the form of a goblin, and who has a few powers one might associate with goblins. A dryad is a spirit who has taken on the form of a dryad and who has a few powers one might associate with dryads. But they're still, at the core, the same type of creature, and the dryad may not have everything associated with dryads.
And the twist is, these spirits can possess humans, giving humans certain powers. And this creates the mythology of vampires, and werewolves, and the like. All mythology in the game stems from human perception of what these spirits have done. Though some argue that what these spirits manifest as is a result of human mythology. It very much becomes a chicken-and-egg question.
But... 99% of this, humanity doesn't know, and about 60% of this, the PCs aren't going to know (or even less if the game master runs a prelude-type campaign). Is this bad world building?
I'm curious to hear your thoughts on the matter.