From "Hell"

"'Our universe has a straightforward space-time geometry, except in odd places like the cores of white dwarf stars. Demons can move around in it without trouble - in fact, they can play tricks with distance and chronology that gave them the reputation of being supernatural in olden days - because their home universe is complicated and variable. Modern researchers have discovered how to get there, but not how to travel around or remain whole of body and mind.'"
-Poul Anderson, Operation Chaos (New York, 1995), p. 221.

That phrase, "...the reputation of being supernatural...," changes everything. These "demons" are the originals of mythological demons. However, they are:

natural, not supernatural;
mortal physical organisms, not fallen angels;
inhabitants of a universe with an unstraightforward space-time geometry, not spirits exiled from Heaven.

When they are killed, their souls, if they have any, will go wherever souls go, to a supernatural Heaven, Hell, Purgatory or Limbo, to another body through reincarnation etc. Steve Matuchek thinks that the "demons" are "...soulless..." (p. 118) but how does he know this? When Steve kills the succubus, its soul, if it has one, does not return home but goes elsewhere.

From now on, the word "demon" should be written inside inverted commas when discussing Operation Chaos or maybe another term should be coined. Their home universe has been called "hell" or "...the hell universe..." (p. 219) because it was previously identified with the Biblical Hell. Indeed, the Biblical Hell must have been based on it. We might call the "demons" "Hellians" or "Infernals" but this would still confuse them with the denizens of a hypothetical supernatural hereafter.