Well after he broke on the Circumfix, Kellhus, for a time at least, seemed to truly believe the lies he'd spun around himself. Later on, he might want to save it for his wife's sake.
I am sort of coming around to the idea that Kellhus himself doesn't actually know why he spared Esmenet, or Kelmomas for that matter. Part of his 'madness' is to not really knowing just how insane he is. The fact that Ajokli is also working his way into Kellhus even before the Golden Room only further compounds the issue, because now we actually have three agencies at work: the Darkness (i.e. circumstance/vestigial emotions), Kellhus' rational mind (the Logos/Thousandfold Thought) and Ajokli.
In PoN, it's pretty clear that the Logos is ascendant in Kellhus, but for a few brief moments. Post-Unification though, I think Ajokli begins to come more to the fore and, to go back to Akka's analogy that I quoted before, the more Ajokli pries open Kellhus soul, the more of the Darkness, the Outside, the madness seeps in. But it's not just Ajokli at play too, because the Thought has Kellhus pitched on a soul-harrowing course as well, lending itself directly to the other two factors rise.
I feel like when Kellhus says he will descend like a hunger, that is
both Kellhus and Ajokli at that moment. As Sorweel experienced when he had the Amiolas on, the line between one soul and the other can be blurry when they are intermixed.