First of all, watch
this video if you haven't. It's not mine, but this guy was right all along, check out the "eye of the corruptor" speech from the javelin questline:
Eye of the Corruptor: Listen. Listen. Listen. LISTEN.
Eye of the Corruptor: You were right to doubt them.
Eye of the Corruptor: The undrowned will join the chorus. Listen to it.
Eye of the Corruptor: Listen. Open the gate. Step into the abyss. Complete the circle.
Eye of the Corruptor: Listen. You know this is the truth. It is but one of a thousand. You have only to listen.
Eye of the Corruptor: Everywhere you look, the stars gaze back. You know their form already. You have but to listen.
Eye of the Corruptor: Listen. She is a threat. Kill her. KILL HER.
Eye of the Corruptor: The queen threatens the king. There is no way to win. Listen.
Eye of the Corruptor confirms it: The Queen threatens the King. In other words, Sylvanas' actions were always about getting N'Zoth ready for destruction. And she's wielding an empty blade vessel that used to hold another old god.
Someone might say, the old god minion could be talking about azshara. Wrong. The "she is a threat, kill her" speech is the very same speech Alleria hears in Three Sisters from the void about Sylvanas.