That is actually not so. If you pay attention to how Tuon is depicted before Sanderson takes over, you'd notice that she's actually a pretty decent person.
In WH Jordan goes out of his way to point out that she's treats her damane well, she believes that no one should be allowed to own damane if they don't show active interest, she also puts on her veil specifically because she had one of her damane beaten unfairly.
In her POV chapter it's also pointed out that her "stern judge" mask is a mask, she's trained actively to maintain it because she worries she wouldn't project authority otherwise.
Also in WH, when she Mat lies to her he wanted to give one of the damane sweets she abandons her "mask" and breaks out in smiles.
In KoD she mentions that she does not believe in unnecessary pain (that when she kills one of the Dark Friends to save her the agony).
But I also agree with you, in that after Sanderson takes over, she just changes. She no longer smiles, and she's not a very good person. There's no longer depth about her. But I don't believe it has anything to do with her becoming the Empress, it's just that Sanderson probably didn't "get" the subtle hints from Jordan's depictions that she's a "closet" decent person among the Seanchan. Also you'll notice that the most ridiculous things about assassins were after Sanderson took over - Jordan probably never had intention to present assassinating as some kind of a national sport, where Seanchans joke about it and tell you to your face they'll send assassins after you - in the books written by Jordan, Seanchans take loyalty seriously, for example in Suroth's POV in KoD she initially cringes at the thought of killing the Empress - and she's a Dark Friend!
But if you create your impressions solely from the later books, then you're right.