There's an internal collision inside Esta's head as he considers his possible later need to swear upon his Law that all he's said to Mariona is true, versus what he is obviously required to believe about this question; and his brain makes an abortive moment in the direction of believing what he ought to believe, about whether he enjoys his job, and then dath ilan --
(actually his model of dath ilan, but Esta can't always tell the two apart, now)
-- yells at him that this is not something dath ilan considers swearable upon his Law, if he's a kind of entity that can immediately believe anything he chooses. And that Merrin won't be fooled about it, either, what with her having already not been fooled.
"I enjoyed some parts more than others. I liked hurting you and the other women I was mentoring. I liked telling you, truthfully, that you could achieve greatness someday, and watching you squirm about that. I was having more fun than you were, I am reasonably sure about that, and had you reached 4th circle you would've had more fun too. But in the end, there was a job to be done. And while it is a requirement of that job that you -- act on your own reasons, in a certain complicated sense," Esta is not, in the middle of this conversation, going to try to explain selfishness as an requirement of Asmodeanism, not least because Mariona always had trouble with that part, "our department is not set up to have its employees' fun as its first priority."