"She saw the Light moving through the cosmos like a ravenous predator. She saw it touch the minds of Azeroth's mortals—a touch that corrupted them forever. She saw generations live and die in invisible chains, bound to a force that granted them fleeting moments of peace in exchange for absolute obedience.
She saw war. She saw the forces of the Light striking back against the Void. She saw darkened worlds burning in holy fire. She saw millions of creatures encased in luminous crystals the size of mountains, sustained by the Light and unable to die. Warriors of the Light were monsters, corrupting and consuming everything they touched."
Sure, Alleria is having a Shadow Vision, but it is close to truth. The Light and the Void aren't good nor evil, they are primal cosmic forces fighting each other and using every tool they can. For example, Xe'ra. We all know what she tried to do with Illidan, but she does the same to Turalyon and Alleria in the audio drama (although these two accepted with no problem). She lied to them about the future, showing a Stormwind-Quel'thalas Alliance, with a high elf Sylvanas.
So, Blizzard is clearly moving to "Babylon 5" look about the Light and the Void.