hm then Idk what people are complaining about.
then again, I didn't follow the entire discussion here.
hell, you might as well say she betrayed the Horde the moment she made a deal with the Jailer way back between WotLK and Cata.
scratch that, she betrayed the world she is living in.
sure, it might be OOC or retcon about Sylvanas' character but that's how Blizzard are going with.