What we got was a moody teenager who blurts out her internal motives ("The Horde is nothing!")

But it wasn't out of nowhere. And that's why I think a lot of times people are a little overreactive toward "this whole story sucks!" when not everyone thinks so.

Yeah, she's been blatantly evil THE ENTIRE TIME, we can all agree with that. But when she was "a moody teenager" it was during a duel in which she seemed to have blatantly broken the rules by casting the nuke with Old God powers. And considering she was being used as a pawn of N'Zoth (same as Azshara), it didn't matter anymore to her whether she had the respect or backing of the Horde at that point. Hell, she was doing some vile shit this expansion before that, which is why it seemed the majority of the Horde were turning against her or at least not agreeing with her way of doing stuff.

So when she breaks this major rule in the duel in front of everyone, it's a slap in the face to most everyone who was still on her side. She knows at that part she's lost their backing, so what's he gonna do, grovel and plead to pretty please be forgiven and be allowed to keep power?