I always like to ask this question when something happens as it tends to help discover motive.
The downfall of the Red Court was inevitable. Harry was the tool, but not the wielder. He is a powerful tool but not moving pieces on the board.
So when we look at the demise of the Red Court, we have to examine who benefits the most in order to understand who might have wielded Harry and what that means for the future.
Nicodemus Archleone - He found them "annoying in the short-term, difficult in the mid-term, and dangerous to any long-term plan". But wouldn't the Red Court benefit from an apocalypse? They could hunt humanity with impunity, no White Council or Venatori to stop them. Perhaps, like the White Court, they weren't the biggest bully on the playground and realised they liked how things were. If the world turned all End of Days they might end up being beaten up by the really nasty bad guys. So it's easy to see why the Red Court might attempt to stop him. Assuming his goal was merely suffering. Jim has sort-of implied that Nicodemus' end goal is to end Reality altogether - I can imagine most of the bad guys who exist on "our" side of the Gates might have an issue with that.
But the Red Court have been destroyed. So is Nicodemus one step further to achieving Empty Night?
What else did the destruction of the Red Court achieve? Well for a start, it seems to have created the right conditions for the Fomor to emerge. Now it seems pretty obvious that the Fomor (like the Red Court initially) were actually preparing for hostilities rather than being provoked/baited into emerging. It does seem like Harry might have forced their hand (just like he did when he started the war with the Vampires), but just like the Reds they always intended to attack the mortal world.
So what does the emergence of the Fomor achieve? Well a more hostile climate for mortals, the probably destruction of the already battered White Council (and their allies), and perhaps a less-restricted magical community. Curiously, a Cantrev Lord of the Fomor would have been a Welsh Lord (as the Word Cantrev/Cantref roughly translates as community - "cant" is 100 and "ref/rev" is town). This indicates a further connection with the Chronicles of Prydain, but more on that later.
Who else benefits from the Reds disappearance? I am obviously leaving out the Grey Council (and Vadderung, Mab and Uriel etc) because their stated reason is protecting humanity which I will take on faith at this point. A number of villains and characters, such as Marcone and Cowl haven't directly stated their intent (unless I am mistaken) to wipe out the Reds yet massively have benefited from the destruction of the Red Court.
Also, it is clear that the Reds were merely fulfilling a function rather than being players. Where the Reds failed it seems likely the Fomor will succeed. Which suggests to me the manipulators are not necessarily the Fomor (although they might be embedded in it) but they two merely serve as a cats-paw of a more nebulous threat (Nemesis/Black Council). The goal currently appears to be a shattering of the status quo and to redraw the lines on the board, so to speak. Only a few characters have indicated more abstract or nihilistic ideals (such as conquering death or ending existence).
Cowl also seems to be an almost cutout of Arawn Death-Lord (from the Chronicles of Prydain). Two great enchanters, both hooded and hidden always, both use necromancy. Arawn was also a notoriously good shapeshifter, and Cowl seems to at least be pretty good at least at some level of magical disguise (zombie in Dead Beat). It wouldn't surprise me at all to find out he had appeared in other books with a different face. Which isn't to say that he is merely, for example, Mac and Cowl. He could also be Cowl, Simon and Mac. Or perhaps only Mac in one scene? We might not see his face for a while if we ever do.
I bring this up because there are a few characters who are apparently bad guys who might not be and good guys who might not be.
A character who has been accused before is the Erlking. He fits the Horned King shape well, but his actions don't. However, it could be that Cowl and the Black Council are only indirectly working with the Outsiders. They might not want an end to Reality, just a New Order. Which might mean that some previously "good" characters might still be bad guys - just not necessarily the worst guys (Old Ones, Outsiders).
So where does this leave us? Closer to the end of the world, I'd say. But also that there are several others who are closer to their own goals who may not have direct association with the Fomor.
Can you guys think of anyone else who benefits from the events of the books, things like the destruction of the Reds but who might not have been overtly backing Harry or perhaps had ulterior motives beyond "saving the humans etc".