He literally says he needs you, that he's obliged to overlook your transgressions, but that really you should've offered yourself up to his honor guard to be killed for being infected. An honorable death to him.
Not so much reasonable as that he really needs you as much as you need him.
If you think about it logically, the moment you meet his honor guard the brain was already breaking out of Gortash and Orin's control.
Say, we and the Emperor die and his guard gets Ketheric's Netherstone. What then? They still can't free him. They weren't able to do so for as long as they've been locked away with him in the Prism. You seem to be forgetting that Orpheus wasn't locked in the Prism alone. Sure, from Orpheus' standpoint dying to his honor guard would've been honorable, but it would also doom the Sword Coast to the Netherbrain. Thousands would die regardless. At least as it is presented in the game currently, the only people who have to die are his honor guard (and possibly he himself if he decides to sacrifice himself to become a mind flayer, which would've been avoided if Emperor used his brain for once) and we get to save those thousands of people who otherwise would be doomed. It's basically an more elaborate 'trolley problem'.
That wouldn't have been in character for The Emperor. He doesn't gamble with his life, which you can see if you try to kill him at the behest of Vlaakith.
Ho ho, my friend! I am aware! It's not my 1st run, on my first run I went along with the Emperor and remained somewhat friendly with him until the end (by end I mean freeing Orpheus). On my 2nd, Durge run I stabbed him there. The end result is the same. He still tells you about Stelmane once you find out she's dead and he still shows up half naked in your dreams later on trying to manipulate you. So much for consequences, eh? The only thing that's different are some responses.
And look, you think it wouldn't have been in character. Okay. Let me present you with this thought experiment: you are the Emperor, a mind flayer that would do anything for survival and freedom (from the brain). Your puppets want to 'betray' you and risk your freedom. What would you do?
a) try to reason with them and talk them out of it
b) go along with it, but make them swear that should Orpheus go KOS mode, they will defend you
- if they swear that, let them do it
x they free Orpheus and he wants to attack you and your puppets want to join him instead of upholding their oath? piss off to the Netherbrain right there
x Orpheus turns out to be a lot more reasonable than you expected him to be, agree to the temporary truce, after all, you need each other
- if they don't, piss off to join the Netherbrain then and there
c) stun them, try to steal the Triune Stone to control the brain on your own leaving them to their fates against Orpheus
d) attack them with the intention of stealing the Triune Stone
e) piss off as you do now and become a potential casualty in the fight should they actually succeed
That is the main reason why so many people are pissed off, including myself, that Emperor just pisses off. There should be a condition under which the Emperor can be convinced to stay. Like damn, Ketheric can be talked into Saren'ing himself (Mass Effect 1's analogy, if you played it you should understand) and it's the most satisfying outcome to Ketheric's conflict to me personally. And yet, Emperor can't be talked into actually using that illithid big brain of his into seeing another solution other than pissing off for a tea with the Netherbrain?
