Xe'ra was revered by the draenei for thousands of years -- leading the fight against the Legion abroad the Xenedar, the Light's largest remaining bastion, and serving as a beacon and a leader to them, as the Naaru Prime, after they had lost their entire planet. And yet when Illidan killed Xe'ra, none of the draenei, lightforged or otherwise, didn't seem to really react, even Velen barely shrugged in response, just moving on as if nothing important had happened. The lightforged were apparently fanatics and zealots -- and yet, they seemed to not care significantly about Xe'ra being annihilated. They didn't object when Xe'ra imprisoned Alleria for an extended period of time abroad their vessel, so they did seem obedient; and yet from Alleria and Lothraxion's conversations about the Void, it seems that the Army does retain its sense of free will, or else Xe'ra would not have allowed him to tell her about such details. Why didn't they attempt to kill Illidan in response? Or at least imprison or punish him in some way?
And why did A'dal attempt to kill Illidan -- if it was against Xe'ra's wishes? The Army were the ones who sent them to Outland to assist Velen and the draenei, according to Chronicles -- and yet they seemed to be unaware that Xe'ra had plans for Illidan, nor did she feel inclined to share her plans with the other naaru whatsoever.
What if Illidan did not destroy her? Or if he had agreed to her wishes? Would it have impacted the outcome of the war against the Legion? Remember, X'era's primary purpose for recruiting Illidan was NOT against the Legion -- it was to ultimately turn him to an extremely powerful and unstoppable Lightforged soldier to destroy the Void, which she told him was a far greater and more dangerous threat by far to the cosmos.