Perhaps we are to draw from this that, in addition to Kellhus simply suffering no rivals, his worry is that Inrilatas would ally with the Consult. Why not kill him though? Perhaps he is worried about the threat of Damnation for infanticide?
Maybe, but I doubt he worries about damnation from any cause. Whats one atrocity heaped onto a mountain? Nothing. So there must be a further reason. I'd guess he needed Maithanet out of the way once he had served his purpose, and couldn't be around to do it himself. Seems a terribly round-about way to do that though. Likely some other reason.
He names two, implying himself as the third.
Whats the context of this? Is it when he is planning the encounter with Maithanet and Kel? In which case, leaving out Maithanet means, to me, that he was having a conversation with Kel/sami, and Maithanet was a ruse for Kel, but he was otherwise irrelevant. Maybe Inrilatas knew he was going to die, needed to die, in order break loose something within little Kel, or to set in motion, inexorably, Maithanet's death, but again so that Kel could reign unopposed for some unforeseen end.
But to what end? Viramsata for the WLW? Setting exactly the path in motion that must carry the WLW forward along the expected path. If its anything like that, then we can go back to Kellhus, and suggest that he needed Inrilatas to carry out his part in the lie that is the WLW's own prescience, so he kept him around... and down the rabbit hole we go.
Inrilatas is only insane in-so-far as he is a sociopath. He was too smart for his own good, he understood too much of everything's base nature to believe in anything higher.
Sociopath yes, but I dont think this is what makes him insane. He appears to knowingly do things that are detrimental to himself
because they are detrimental. His complete lack of self-preservation makes him insane.
Also, consider Kellhus' reaction:
"For some reason Esmi suspected Maitha of sedition," Kellhus said without the least whisper of remorse or concern, "and so called him to account before Inrilatas. The interrogation went wrong, horribly wrong, and my brother ended up killing my son..." He looked down to his haloed palms, and Proyas found it curiously affecting, the contrast between his tone and his manner. "I know little more than this."
If he wanted to know more, he would. Thus, he is either intentionally deceiving himself or is feigning deception. My bet is on it being a ruse. He knew the Empire would crumble, but it has already served it's purpose so it's irrelevant. There's no plan for a return.
Feigning deception, but not because he doesn't care for the Empire, but because he knows that it must necessarily undergo a painful transition from the Anasurimbor rule to the rule of another. He will just as easily conqueror it on his return trip as he did Sakarpus. After all, they had the chorae horde and were defenseless. I think it will just make his job easier to have all his enemies in one place, sitting in his throne room, rather than having to hunt them all down in the wilderness.
So, in a way, yes, his own empire as The Ordeal knows it, is forsaken. What good is a country filled with only the very old, the very young, the sick and the impoverished? Better to come home to a replenished country filled with new subjects, healthy and ready to fight, after a year or two of coaxing into the same fervor that now grasps The Ordeal.
The death of the old empire does not imply there is no plan to return home, just that the home will be quite different than his men may suspect (if any of them even make it back).