Originally Posted by
Aucald
In this context, I would say the most salient aspect of the argument isn't whether the majority of the Desolate Council was or wasn't loyal to Sylvanas or the Forsaken, it is actually that Sylvanas herself didn't know the answer to this question and ordered their massacre regardless of sure knowledge. That, we actually do know from the text of "Before the Storm" - Sylvanas suspects the Desolate Council members who didn't return early from the Gathering (with their hearts duly hardened), even Velcinda/Elsie isn't safe from this suspicion as Sylvanas suspects Elsie well before Calia's revealing, when she simply acknowledges Anduin Wrynn from afar.
Now, as for the question of whether or not that makes Sylvanas evil, or a monster, that's more subjective and harder to pin down. Sylvanas doesn't know or understand Anduin in the same way we are permitted do (e.g. she can't know his thoughts, or inner drives, as they're presented in the narrative), and given the state of the political reality between the Horde and Alliance she has precious little reason to trust him or to take him at his word. Anduin, too, has to keep up the appearance of strength - and while he *desperately* wants this whole Gathering thing to work out (largely from a sense of spiritual altruism on his part, being a Priest of the Light) he can't make direct or indirect concessions to a party hostile to the government he represents.
Of course, Sylvanas realizes that Calia's folly isn't a product of machinations on Anduin's part pretty early on - it was her initial theory, but a quick view of the facts of the case makes her see (rightly) that Calia is acting alone and out of misplaced compassion for "her people." What does Sylvanas do with this knowledge? She capitalizes on it quickly, seeing the chance to kill a possible usurper to her throne without invoking an incident as well as liquidating possible defectors or future agitators within her own ranks (in the form of the remaining Desolate Council members on the field).
Sylvanas takes advantage of the situation, in other words - for her own gain. Does that make her evil, or a monster? Maybe so, or maybe not - I would say it makes her amoral at the least. An argument could be made that allowing dissent into the ranks of the Forsaken would be worse than purging it, especially in light that the future holds only more conflict (by Sylvanas' own designs) with the very people that the Desolate Council wants to make amends with at the Gathering. That in purging them at Arathi she is sparing them a worse fate later on, when they would have to directly or indirectly raise arms against the friends, family, and loved ones they were trying to reconcile with.
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The Forsaken can't have a different leader - rightly or wrongly, the Forsaken as a people derive directly from the person of Sylvanas (in both the figurative and literal sense, as the case may be). Though I doubt the majority of Forsaken would ever rise up against their beloved Banshee Queen, if the situation were to change it would still make no difference - Sylvanas is the quintessential hydraulic despot, and her coalition (herself, the Val'kyr that are bound directly to her, Nathanos, and the RAS) is duly small and held entirely in her thrall. She is, quite literally, the means of production for the Forsaken race. Everyone else in the ranks of the Forsaken are a powerless selectorate with no real selection, and no means to force the issue. As it stands, Sylvanas would destroy her own power base (the Forsaken themselves) before she would capitulate to a replacement.
Sylvanas does permit Forsaken to leave her service, but not to the Alliance. Of course, the same could be said for the whole Horde, really - an individual Orc may opt not to leave in Ogrimmar and be a mercenary or a sellsword, but working for the Alliance is still going to be a death sentence if it is discovered by other Horde members. As for the Forsaken and death - no, we really don't know what happens, but that doesn't really change their lot as we don't what happens with living individuals either, and the Warcraft universe is one where literal demigods can interfere with or wholly shape an afterlife scenario directly.