So the opening cinematic from Warcraft 3 The Frozen Throne says it all. Hes the one who ushered the Naga into the world.
In the War of The Ancients novel, its suggested that several being more powerful than Sargeras were augmenting Illidans Spell to shut down the Well of Eternity. Why would he willingly stay behind at the Seat of The Pantheon?
The Twilight Cult talks about the unseen victory at the Seat of the Pantheon, Illidan, working for the Void may have consumed the power of Sargeras in his weakened state. Illidan, like the Void really wanted the Legions end.