We know Kil'Jaedan is the Deceiver.
Might he be deceiving Sargeras?
He gave up Argus, according to the books, to conquer and rule worlds, not be Sargeras' whipping boy for eternity.
If he hopes to steal Sargeras power, his only competition, Archimonde, is dead and he has had thousands of years to master Warlock skills like draining souls and learn how Sargeras originally defeated the demons.
We don't know if he knows what Sargeras is, or how much he knows about Titans, who were destroyed before he joined Sargeras.
He would need both information and allies to distract or weaken Sargeras enough to destroy or drain him, allies like the Old Gods perhaps? The race of whatever Elune is?
(( The not serious speculation just for fun ))
It has always been assumed that it was the Old Gods who whispered to Azshara and offered her life, but Kil'Jaedan certainly knew about her, and had the ability to transform her. She is entwined with Illidan who is an enemy of Sargeras and a useful tool but not one that can be weilded directly by Kil'Jaedan.
It was never explained in Warlords how Khadgar knew things he shouldn't have, so Kil'Jaedan could have been that answer though there is zero evidence either way.