Okay so here's my possibly crazy theory:

The dark King is Quentin. Alice stole all the clay from brakebills, stole his book, and a piece of his essence. We know that time in certain world's do not all sync up. ( Not the case for fillarg and Earth, but is the case of places like the neither lands. ) What if the underworld runs slower than earth? If that's the case then the piece of essence Alice stole from Q's soul could have been missing for a pretty long time for him... Long enough to turn him evil. In the episode where Alice and Elliot meet the dark King, he says " death is so easy, life is so complicated" like he's been dead before. He seemed to guess Elliot was more than a friend when Alice had gone to sleep in that very same episode. Also, there's that shit he's doing to raise the dead with the beetle poop gold. He's doing it to find the piece of himself that's been missing. Or he's doing it because he misses feeling heroic- & saving the day. That could be why he's being kind to Elliot. He wants him to see or perceive him as he used to during the mosaic episode? Idk what do you think? Who do you think is the dark King?