I think people are misunderstanding the Emperor. When your teacher says that if everyone behaves they'll get a pizza party and the class starts to act up, they'll remind everyone about the pizza party. That is an act of manipulation. Manipulation isn't inherently malicious and I think that's what he considers himself to be doing. He knows that if he shows up in your dreams as a Mind Flayer you won't trust him, so he hides that from you. But no matter how rude you are to him, if you trust him in the end he will destroy the nether brain. At the end of the day, regardless of what the nether brain says or what the player thinks, he's true to his word.
In regards to him betraying you if you side with Orpheus, I agree that that's not very cash money of him. However, Mind Flayers are incredibly intelligent. You showing up with the Orphic Hammer isn't really a surprise to him, so he's most likely thought through what happens if you choose to free Orpheus. He's wrong, of course, but he's most likely making this decision so quickly because he's already got it planned out. He may not actually be wrong since Orpheus may react differently if he woke up to an actual mind flayer instead of just a tadpole.
