The Apostle had a point. One would not expect that. And the Alpha Legion would not expect it either, if they were really behind it. Though there was the possibility of the infiltrator close to the Apostle did know of his intentions. In any case, if Setesh did go instead of some kind of kill-squad, or worse, of the Word Bearers it would be cleaner — assumably. A follower of Tzeentch who threatened a world of Tzeentch with a possible invasion in retaliation was borderline foolhearty. A trait that wasn't too rare, though.

"That sounds logical. But what do I have to gain in doing this task for you against one of my own?" The sorcerer used the word "own" very loosly, here. He did not in particular mean it. But he figured it was a good word to use in this situation.