More time travel could be involved, but everyone speculating about the identity of the Demon King, and whether or not it's Kail, should remember two things. The first being that there's another, parallel world in play, the first world that was ruined. And the second being that either time traveling gives you a soul identical to a demon, or perhaps it's the other way around, and all demons have the doubled souls that time travel can give and Zeal was attempting to create.
Maybe the Demon King is a Kail, and maybe he's not, but some of you are overlooking that he could just as easily be the Kail from the first world, potentially. He may not be a Kail from this world.