I did kill the mind flayer just now on my second run. It ends the game. That's why I am here.
My point is that, as a good aligned character, I have no choice at that point -- I must kill the mind flayer, then myself, and end the game. I have sworn an oath to do this and the game gives me no other option.
(1) My first and primary goal is to rid myself of the mind flayer tadpole.
(2) Other than an offer from Raphael that I cannot accept, I have made no progress on that quest -- every lead was a dead-end.
(3) I am not surprised to learn my "guardian" is a mind flayer since he is communicating with me telepathically, refuses to identify himself, and gives vague answers to every other question.
(4) Obviously I cannot trust a person who won't identify himself and who has been deceiving me about his appearance.
(5) Indeed, my "guardian" is encouraging me to feed even more of my brain to the worm -- exactly the opposite of my primary quest.
(5) I know the Githyanki will kill me, but I also know that they have the same enemy as I do -- the illithid. I can at least trust them in that respect.
(6) I cannot carry my infected brain into Baldur's Gate. If it is not removed now, I must die -- just as Lae'zel and Nettie advised in Act I.
Every "good" run must end in the Astral Plane before reaching Act III. If we want an immersive experience we cannot use the excuse that our characters know the future, or are aware that they are living in a game-world.
**As a corollary, this means not recruiting Minsc (unless they made him evil). Surely Minsc from BG2 would not join a party of evil half-illithids.