I'm really starting to believe that the man in Kellhus's dreams is Kellhus. I don't know that he want to shut the Outside, the more I think about it. He wants to dominate it. This killing the 100 and him becoming the sole God of the Outside, the Absolute. Maybe just him and Onkis, can't get rid of the head on the pole behind you. Now, mind you, killing the 100 would still essentially stop damnation to the extent it's become on Earwa.
That's the spirit!

Think, if Saubon could do it and he knows
nothing of the Outside, what could Kellhus do? Speak directly to himself, perhaps?
Further, all the things we joking, or not jokingly, ascribed to "shadow Moe" could easily apply to "future Kellhus."