Azshara: We have a bargain, then. I will bring both fleets crashing to the ocean floor, and your champion will deliver the dagger to me.
Sylvanas: And in turn, you will have the key required to free the Old God from his bonds… and leave him vulnerable.
Azshara: You wound me, Warchief. After all, I am as dedicated to my master as you are to your subjects.
Sylvanas: Indeed. Just be certain that once you have what you need, you dispose of your guests. Let none of the “heroes” escape.
Azshara: I admire your ruthlessness, Windrunner. It seems our interests are aligned… at present.
Sylvanas: At present.
Azshara: Treacherous Banshee! Do you think I am blind to the darkness you seek to unleash?
So basically, they were working in tandem to get N’zoth out of his prison, and vulnerable enough to finish him off.
Guess Azshara wanted so to become the lone ruler of their armies, while Sylvanas is doing so serving this vague “Master” or Death Entity.
Also, the fight dialogue points at us allying with Azshara:
Submit, Azshara! Beg forgiveness for your failure! Your betrayal!
Insolence only prolongs your agony! You will break, or I will carve out your defiant tongue!
Impudent underling! You will… address me… as queen!
More likely, Xanesh… Azeroth’s champions… will be the ones… to silence you.
Release me! Only I possess the means of defeating N’Zoth!
Mind your tone, whelpling. A queen keeps her promises. Listen closely, as I will only say this once.
For all his bluster, N’Zoth was ever the weakest of his kin. One vestige of their power yet remains that can be turned against him.
I would have driven this blade into his foul heart myself… had you not disrupted my plans.
No matter. Xal’atath is yours, mortals. Strike true. Should your first blow fail, you will not survive to attempt another.
So…the one Kerrigan’d wasn’t Sylvanas. It was Azshara.
Or maybe i guess its more like Illidan in TBC.