Even more so than with vampires, entire volumes can and have meen written on the subject of demons. The concept is a broad one, but rises out of a mixture of religious world views, as well as popular and cultural myths, folklore, and superstitions. The idea of a lord of an underworld is found in many hierarchies of gods and goddesses: the Romans had Pluto, the Greeks Hades, and the Norsemen had Hel, to name a few. In addition, a vast array of mischievous spirits, faeries, wood sprites, poltergeists and so on populate the religious and mythological landscape.
In the Judeo-Christian system, Satan (or Lucifer) has his subordinate demons whose purpose is to harry humanity with aggravation and temptation. In the Old Testament, Satan was an angel whose occupation was to test humanity's loyalty to God. Only later do we have the tale of Satan's rebellion against god and the casting out of Satan and the angels who had sided with him. After which, of course, they became known as Demons.
Later in the history of the Christian church, around the twelfth or thirteenth centuries, there was a general consensus that demons were actually the old gods and goddesses, not only lords of the underworld, all of whom had animal incarnations of some kind. At baptism Christian souls were commanded to renounce 'Thor and Odin and Saxnot and all evil beings that are like them.' But the older notion that demons were fallen angels serving Lucifer is the belief that proved more resilient and became the more traditional definition of demons in Christian teachings.
However, in the mythology of BtVS, the traditional story of creation is not considered a myth. The demons in this series are based not on Judeo-Christian and Western tradition, but are more influenced by Eastern traditions, and to some extent by the works of the prominent horror forefather, H.P Lovecraft, whose own mythology seems to have been based largely on ancient Sumerian demonology. Lovecaraft's fiction included references to a race of 'Old Ones' or 'Ancient Ones', who existed on Earth before humanity and who were constantly trying to regain control of our world. Various acolytes and half-demons laboured to open the gates and let the old ones through.
Also, as previously discussed, the vampires in Buffy are corpses inhabited ny demons, and thus it follows that in the Buffy mythology demons are more often described by their function than by the form they have currently taken.
Ms Calendar: "I knew this would happen sooner or later. It’s probably a mischief demon, you know, like Kelkor or-"
Giles: "Its Moloch."
Ms Calendar: "The Corrupter."-"I Robot, You Jane"
Cordelia: "Ew. What does this do?"
Giles: "It, um, extracts vital organs so that it can regenerate its own mutating cells."
Cordelia: "Wow. What does this one do?"
Giles: "It elongates its mouth to engulf the head of its casualty between its teeth."
Cordelia: "Ouch. What does this one do?"
Giles: "It asks endless questions of those with whom it's supposed to be working so they're not getting anything done!"
Cordelia: "Boy, there's a demon for everything."-"I Robot, You Jane" This stated function is generally some kind of static goal, from a mindless reflex to lash out and destroy to a more sophisticated need to dominate and control. This is a limitation of demons, as opposed to the more fluid and multidimensional "living of a life" that humans engage in.
"I really like this feeling. Sort of mindless need to destroy. This anger and rage."
-Demon Giles, "A New Man"."You feel smothered. Trapped like an animal. Pure in its ferocity, unable to actualise the urges within. Clinging to one truth. Like a flame struggling to burn within an enclosed glass. That a beast this powerful cannot be contained. Inevitably it will break free and savage the land again. I will make you whole again. Make you savage."
-Adam, "The Yoko Factor".Angel: "Things change."
Spike: "Not us! Not demons."
-"School Hard".As demon inhabited corpses, vampires are evil, and they remain evil, despite the assertion of the Judge, who says of Drusilla and spike that they stink of humanity, sharing affection and jealousy. In a sense then, what makes them human is their capacity for feeling emotion; what makes them demons is their inability to change, their emotions don't grow or lead to good as human emotions can. They are what they are and they remain that way. And if evil is the inability to choose good, or feel love, for instance, then perhaps that explains the fury Angelus feels when he contemplates how completely he was changed when his soul was restored. An interesting point is that Spike, unable to do harm because of his chip, falls in love with Buffy. Does this make him more human and less evil? Is Spike is a demon, then why is he prepares to give up his evil ways for Buffy's love, and how can he feel love at all? It is possible that by removing Spike's power to do bad, as it were, this has somehow changed Spike for the better. Maybe, maybe not.
Buffy: Please! Spike, you're a vampire.
Spike: Angel was a vampire.
Buffy: Angel was good!
spike: And I can be too. I've changed, Buffy.
Buffy: What, that chip in your head? That's not change. Tha-that's just ... holding you back. You're like a serial killer in prison!
Spike: But I'm not ... like that. Something's happening to me. I can't stop thinking about you.
Buffy: Uhh.
Spike: And if that means turning my back on the whole evil thing--"Crush" However, there are also good- or pehaps a better word is 'neutral'-demons in the Buffy universe. Whistler is one such demon, and he even describes himself in terms of his function:
Angel: "You're not a vampire."
Whistler: "A demon, technically. But I'm not a bad guy- not all demons are dedicated to the destruction of all life. Someone has to maintain a balance, you know. Good and evil can't exist without each other, blah blah blah."-"Becoming, Part One". The introduction of a character like Whistler suggests that Joss Whedon's universe, at least, there seems to be a continuum running between the opposite poles of human and demon; in other words, some demons are more human than others, and some humans behave more like demons. Or, to put it another words, what makes us human is our capacity to change, to feel emotions, to choose good over evil. As Buffy says to Ford, the terminally ill young man who wants to become a vampire so that he won't 'die':
"You have a choice. You don't have a good choice. What's behind door number three is pretty much a dead fish, but you have a choice. Your opting for mass murder here and nothing you say to me is gonna make that okay."-Buffy, "Lie to Me". Having chosen such a versatile approach to the demon, Joss Whedon has provided vast opportunities for more stories about demons, what makes them different from and similar to humans, and the choices they make or fail to make in the ongoing battle between good and evil.