The Naaru are servants of the Old Gods!
What, you want something to back that up?
"Without the void, the Light cannot exist."
It all starts long ago, with Sargeras, Champion of the Titans, preserver of justice in the universe. He had long fought demons, and became weary. The Old Gods, which he had also fought in a cataclysmic war, sensed this, and contacted him, offering him power, the power to control their most malevolent creations, the demons. Sargeras accepted the offer.
But he was just another chess piece in the (to say the least) complicated scheme of those who were old when the universe was young. Out of the Void, they created the naaru, the perfect servants. As Sargeras corrupted the eredar, the Old Gods sent the Naaru (transformed from their original form into a more fitting shape, as agents of the Holy Light (in truth, the Light has nothing to do with the Naaru, it existed before)) to warn one of the eredar (Velen) about the Dark Titan's deception. The Naaru pretended to be being of the Light and that they helped the draenei people. In truth, they protected them from the Legion in order to use them to finally conquer Azeroth. When Elune stumbled upon the draenei, the Naaru turned her away, "converted" her.
Some of the Naaru, though, out of their masters' control, actually turned into actual servants of the Light (such as A'Dal), while many still serve the Old Gods with fervor. Kael'thas, the demon-tainted prince of the blood elves, actually found out (an Old God told him so) about the true identity of the naaru, and kidnapped one, M'uru, who had turned to the Light, and forced him to eventually turn back into his original form.