And Illidan killed scores of innocent people to convince the Legion to induct him. It's strange WoW has this extreme the ends justify the means mentality, but that's what happened.It was, because even though everyone wasn't going to the maw yet, Zovaal showed her the maw when he made his pitch. He led her to believe every forsaken was going to the maw even though it was just because he already dominated her soul because she was killed by frostmorne. Part of Sylvanas was already in the maw because of Frostmorne, just like Uther's soulshard. It's why Uther was getting visions of the Maw.