I agree with PC Oquelin. I don't think Winnowill has left her "real character mode," but that she's been slowly getting worse over time. Her madness is progressive. In Blue Mountain, when she was still at least partially under Voll's restraint, she was better. After Voll's death, she got worse. Leetah's aborted healing attempt made things a lot worse-- and then she had 10,000 years to "refine her evil," as PC says-- or to dwell on her injuries and perfect her bitterness. Then she learned with Grohmul Djun the intricacies of domination in ways she hadn't before.
I think the way we see her now is the author's conception of what such a character would become when all constraints of physicality are removed-- and then she is imprisoned against her will in the body of her jailor.
She's reached the pinnacle of where her choices have led her. Now it's up to Rayek to try to lead her back.