3. Endgame choice. Enough has been said about his entire character falling apart the moment we choose Orpheus, it's appalling and completely out of damn character. And most frustrating of all? He can't be stopped. You can't toss a damn spear in the portal he goes through in hopes that maybe he gets hit? Not a chance to talk him out of it? "You choose Orpheus? KKTHXBYE" like what is this writing?
I can answer this one. His entire purpose is survival. Guess what he's not going to do when Orpheus is released? That's right, be allowed to live. So to him, it's better to cast his lot with the Netherbrain in hopes of getting another chance to free himself in the future. He's not looking to be a martyr or serve the greater good. He's in it for himself and you and your party members are just a means to an end. He doesn't even have a soul.
Okay, I concede, he doesn't have a soul. But that entire 180 turn is just so abrupt. In most situations the characters usually try to reason with you, talk you into or out of certain decisions.
The moment you say: "Let's free Orpheus!" he just dunks out immediately, instead of trying to reason his way out of this situation. Before his excuse was that "Orpheus hates mind flayers and mind flayers in the making, so he will attack on sight!"
And surprise surprise, the moment Orpheus is freed, he is actually pretty reasonable, aside from few accusations. The githyanki egg was indeed a trap (even if we kept it stashed away the entire time) and it's quite understandable he'd be pissed, but killing his honor guard? Who were provoked in the first place by the Emperor? And then he claims they would've freed him. Somehow. A deed they weren't able to accomplish for 1k years. Also, without the Orphic Hammer or any other means of breaking those chains. Yeah, nice story buddy.
Regardless, all Emperor had to is wait it out and see if Orpheus' hatred towards him was stronger than his desire to kill the brain (both of 'em share a common goal after all, they just go about achieving it differently) and free his people. Maaaaaybe if Emperor proved himself, he wouldn't be killed just like mind flayered Karlach and Tav aren't. Like shit, give us a DC 99 persuasion check to convince those 2 to work together. Anything really.