Point is without factual context its all speculation based on personal perspective, a lot can change over a decade especially the circumstances of her entanglement, like why didn't he just truly enthralled her completely, then there would be no evidence of his tampering otherwise, he had a decade after all. Just because he showed you this scene, in response to you openly vilifying him for his appearance no less, is just as much a threat and a confirmation of the player's choice of bias. I'm not convinced until someone unveils the emperor's evil master plan for world domination which hasn't happened yet.
Lying about personal history by omission is a really low bar for judging someone's overall alignment in my opinion, since most of the companions did it in varying degrees and most everyone is willing to forgive those in a heartbeat, even the life threatening ones.
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Once a vigorous and formidable politician, Duke Belynne Stelmane recently suffered a seizure that left her with a partially paralyzed face and slowed speech. In truth, a mind flayer provoked the duke’s “seizure” when it took mental possession of her. Now Stelmane wages a silent war against the mind flayer’s influence, biding her time until she can find a way to signal for aid or regain her will. Not even Stelmane’s aides are aware of her secret struggle, though they cover for her as best they can.
The version of events he reveals to you when you press him is confirmed by sources outside of the game - player bias or not. Now, if you ask me that doesn't sound very amicable. In fact that sounds like a type of thing even a decade of servitude couldn't smooth out into a fruitful friendship. But even if we didn't have outside sources confirming this, the Emperor would still be a lying manipulator who creates different versions of events in order to get you to do and believe what he wants.
In regards to your second point. It's not a lie by omission. If I tell you that I'm friends with someone when in reality that someone has been my prisoner for the last 10 years that is not a "lie by omission". That's just an outright lie and a pretty blatant one at that. Also whereas your companions open up on their own and confess their secrets to you eventually, the Emperor only reveals his lies when you show him enough hostility for him to realise that the milk is already spilt and he has to try a new approach to get you to do his bidding. If you treat him nicely he will never tell you the truth about Stelmane because he's a manipulator who only tells you the bare minimum he needs to ensure your cooperation.
So yeah, if you think the Emperor is a good trustworthy guy knowing all this I have a bridge to sell you.
