Just throwing this out there: the Dunyain are not above murder. They kill defectives, or send them to the unmasking room, or whatever else. If Moe came back to them with knowledge or experience enough to warrant worry within the general Dunyain ranks, why not just kill him? They are many, he is one. Shouldn't be a problem, right? This raises two possibilities, in my mind.
1) The Dunyain truly believed themselves to be the last remaining vestige of humanity in the world, secluded as they were. So, in effect, they believed they were killing him by exiling him. How long could he survive? There's nothing out there, no other people, no women to procreate with, no way for himself, alone, to assault Ishual to 'pollute' them further. He had become a danger to them, specifically, because of the knowledge he had gained while outside Ishual. They weren't afraid he'd tell anyone about them, because they truly believed there was no one else. His perspective was the danger. Maybe he 'broke' like Kellhus did when he left Ishual.
or
2) Moenghus had become powerful in a way the Dunyain did not understand. Meaning either he had learned something that could overcome multiple Dunyain, or he made them believe he had learned something that could overcome multiple Dunyain. Whatever the case, the result would have been the same: exile rather than risk further losses/corruption/whatever. Which then leads to (1) above anyway. Fine, exile him, he'll die anyway.
I don't really think he learned sorcery or had some tekne or some other bullshit like (2) might imply. I lean heavily toward the first option. He'd been corrupted to the point where the other Dunyain felt his presence could no longer be tolerated for fear of derailing their mission. Simple as that. The Shortest Path is exile: he's gone and no-one had to die to try to subdue or kill him outright. Problem solved.
Until the dreams...