I personally have a feeling that there's a subtle civil war that's occured within the Legion, with Sargaras out of commission (due to his failed gamble with Medivh), Kil'jaeden took over.
We know that TRUE demons can't be killed, not without going to the nether and severing their connection to it. But they can be STOPPED if part of their body is held and sealed (such as with Sargaras and other demons). Kil'jaeden is leading in his master's absense, but in that, he may not know EVERYTHING that Sargaras had planned.
It's been a few decades since Sargeras was sealed, and as such, I think that Kil'jaeden isn't interested in bringing back his master for ANY REASON. What he wants is his master's power. But he needs to know how to extract it without it overwhelming himself. Thus, he needs the only Demon Hunter who has gone full demon while being free of influence from the Legion - Illidan.
Thus he takes a dead Naaru, only a few minutes from going full void-monster, puts it in stasis and then throws it into Azeroth with all the pomp he can, so that we find a way to rez Illidan so he can capture Illidan and get the information from him. He used the corpse of the Naaru to lie to us about there being and "army of Light" and the titans being dead. And the reason is that he doesn't really care what he has to say to get us to do his bidding.
But all this being said, I think Kil'jaeden is unaware of the mistake he's making.
Because I have a feeling that Sargaras didn't want to come to Azeroth for a foolish reason, or even a "need to get to the old gods, because he was corrupted by the old gods". He wanted to come to Azeroth to fortify it...because he created the weapons on it...and they're the ONLY THING THAT CAN KILL THE TITANS.
I think we're going to have to kill the Titans. Because we're "flawed" and they don't want "flaws". Sargaras didn't start the legion to raze the universe, he started it to PROTECT the universe. The Titans went everywhere, slaughtering indiginous species that were "flawed", locking away the natural elemental spirits, inserting their own programmed species, and if that proved impossible, destroying the planets they were on.
Sargeras rebelled - not because he was "influenced to evil" when he first hit Argus, but moved to COMPASSION. Here was an ADVANCED society. They knew magic, technology, travel between worlds, and being as they're not "built like the titans", they were also considered FLAWED. Sargeras gave everyone on Argus the option - join him or die. But Valen didn't understand that the DEATH was going to come from the TITANS THEMSELVES. They fled the system, becoming the Draenei, meeting up with the Naaru (beings of light with a dark secret - they are also beings of incredible DARKNESS when they die, until once again, they die, and are reborn as beings of light) and trying to hide from the Legion...without knowing that the legion was the one thing keeping the titans at bay.
The Legion spread, but so did the issue of corruption within the legion. Kil'jaeden was absolutely not interested in Sargeras' plan to be his "army of light", because he really just wanted to take over everything, and rule the universe himself. And once Sargeras got himself locked away, Kil'jaeden made his move. All the actions from The Burning Crusade and Legion are Kil'jaeden trying to take over the weapons on Azeroth so he can rule. But one thing is in his way - he doesn't have Sargeras' power. This means his control over the legion is not absolute, and as such, there's always a chance he'd be betrayed.
There's only one way I think this can resolve:
Kil'jaeden is going to figure out that he cannot handle the power of Sargeras (possibly due to genetics), and to save himself, will put that power in Illidan. This will awake Sargeras within Illidan, and Sargeras is going to stand up and go, "You're a freaking MORON, we needed them as ALLIES!" and just SEVER Kil'jaeden from the nether.
And then we're going to have a massive meeting of the minds - Sargeras is going to explain what is going on in the universe, and we're going to have to make a choice.
Join the Legion...or die.
And we're going to join.