I know it’s been said a few times in this thread already, but Sylvanas and Daenerys pretty much have the same writing issues in BfA and season 8 respectively.
Female leader who suffers some pretty horrific stuff, but through the virtue of their own will (and some helpful magic), manages to free herself and others from captivity. Those people aren’t the ones she was naturally born to, but through her deeds, they flock to her as the Queen they choose to follow. She leads well, but certainly not perfect. She has many dark and cruel moments, and people do have a right to fear or hate her for those. She needs allies for her goals, and joins arms with foreigners who her former people certainly had a troubled relationship with, but that she manages to make work.
Then as the story progresses, she suddenly gets to a point where those people she has allied with are the ones that the writers want to be “the good guys”, and she’s a major player standing in the way of that. She is suddenly written to have bouts of irrational emotion and jealousy for another female character that threatens her power, and all of this culminates in a story that turns her on a heel into an outright fascist who burns down an entire city of civilians.
We’re told that this was always her character, that these small moments from her past are why we should have expected full-scale genocide all along. The moral of the story seems to be that her natural state of existence as crazy and evil was inevitable, and we were just waiting for the coin to drop. All that’s really missing from Sylvanas is that instead of her Jon Snow completing his turn against her after starting to doubt, and killing her, his story just never continued.
The big issue with both of these characters is that foreshadowing and the potential for great evil doesn’t entail doing great evil. It has to actually be built up to and given reasonable motivations, rather than existing to provoke the other characters to hate her and push her out of the good guy team.
You can’t jump from Daenerys having major doubts about executing two military leaders to city-wide genocide, just like you can’t have Sylvanas jump from explicitly not wanting wars to keep a target off of her back to city-wide genocide to spark a war intentionally.