@Nathasil you're absolutely right. Nice explanation, she wants like a Scourge 2.0 and the undead wanted revenge to the scourge and living because of Arthas now he is long gone and Sylvanas is only doing everything for her personal goals and i can see the undead going against her. She ordered her pawn Nathanos and his dark rangers to fill it with blight once she was done talking with the Alliance to destroy Lordaeron, so they couldn't get Lordaeron but doing this she shows how much she cares about this place and the undead: nothing.