Sylvanas probably would've done away with them differently. All Anduin did was make it easier for her.https://twitter.com/soulsobreezy/status/1008198877164367872 …
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But what would it have been? That's the fun question to ask when putting Alliance cynicism aside. Consider that the Forsaken is both a race and a nation that's not even ten years old.
Sylvanas is such a fickle planner she'd do something that would probably have the Desolate Council be turned on somehow (shut it Bob). Make them the enemies rather than make her out to be the enemy. Potentially she could also make it look like the Alliance killed them.