After WotLK and her deal with the Val’kyr, Sylvanas was inevitably going to become a villain because her motivation stopped being about the continued survival of her people but rather her own at any cost. And even then, it could be argued she never actually saw them as anything more than a means to an end even before WotLK given her dialogue in Edge of Night.
The warning signs were already there, but the Horde playerbase kept chalking it up to her being ‘gritty.’ Indeed, the more she seemed to step over the line, the more the Horde playerbase cheered it as ‘strong leadership.’
Heck, most of the Horde playerbase cheered her actions all the way up to Saurfang challenging her to Mak’gora at the gates of Orgrimmar and there are still those even now who are trying to rationalize what she has become.
Why? Personally, I think it’s because she made the Horde playerbase feel powerful which lead to them ignoring the obvious direction her character was taking and continually rationalizing away her more questionable actions even when it was obviously not in the best interests of the Horde.
I think they knew full well where her future could lead, but they were only too happy to turn a blind eye to it when they thought her actions were helping them WIN.
I remember reading the forums after the Burning of Teldrassil and saw just how many Horde players were downright gleeful about their rationalizations thereof. They didn’t care what she did as long as they thought it was benefitting them, they only became outraged when her actions and theirs had tangibly negative consequences applied to them.
I understand that many Forsaken like being morally grey or even bad and that it’s one of the big draws for some to the faction, but in a game where good and evil is more clearly defined, when people go too far or cross the line one too many times there will inevitably be consequences for it.
You can’t repeatedly commit evil acts and then be mad when you’re treated as evil. You can’t have your cake and eat it, there has to be a balance unless Blizzard decided to go the route of creating a third faction which is unrepentantly evil like with Warhammer.
If anything, her downfall is probably the best thing to ever happen to the Forsaken. Not only do they finally have the opportunity to decide their future for themselves, they’ve learned an important lesson on the dangers of blind loyalty that they will hopefully carry forward into the future.