Is this not true for all players too who immediately upon learning his past start judging him and condemning him just because he is a mindflayer who prefers to keep his story out of the way? So yes, he is absolutely right to hide details of his past because as you see majority of players immediately convict him of crime, despite him being a victim in it.
Second of the Emperor is not a malicious whatsoever.
[list][*]This is proven numerous times because nothing he does towards us is malicious in any way, unless you become malicious towards him.
Even if you consider someone words malicious, which I think is debatable in this scenario, how a person reacts can tell you a lot about them. My character told the emperor the truth, that she would not trust him because he is an expert at mental manipulation, when he tried seduction. At which point he reacted by forcing the vision of enthralling Stelmane. And in my opinion that is a sociopath's response; a sociopath who for a moment got angry - because his manipulation failed - and let the mask slip. Because even if he considered my character's words malicious, I think any non-evil person would simply call them out on their behaviour. Ignoring consent and threatening enslavement is a giant waving red flag.
I also think there is a hinted possibility that when the emperor says he 'finessed his methods', he means his mind thralling powers. If you react to his "seduction" by telling him to stick to business, he'll simply reply "good instincts". But there is a weird narrator line afterwards, where she says your character feels disappointed that the mind flayer was quick to abandon its attention on you. But why would the narrator be able to tell how your character feels? Unless the situation is similar to that of the of dark urge, where it is outside influence. Of course, it could be simply Larian writers overstepping their boundaries.
