Funny enough, the reason for Sargeras turning to the dark side has changed a few times over the years.
- Once it was said that the Eredar were responsible for his corruption but they changed that when they wanted to add the Draenei back with the Burning Crusade.
- Then they gave it more of a philosophical reasoning. The constant darkness of the universe eating away at him, slowly but surely making him believe that the universe wasn’t meant for their orderly plans and that chaos was the way to go.
- Finally with the Chronicles they gave that idea a bit more substance.
In the grand universe of Warcraft there is the power of the Void and these so called Void Lords seek to devour all matter and energy in the physical universe. They saw the titans do their thing and became envious of their power. First they tried to corrupt these fully grown titans, but quickly found out that that wasn’t going to work. Instead they set their sights on the ones more vulnerable, the ones still slumbering inside of planets. Like the titans, the void lords had no idea which planet actually held one of these souls so they pooled their powers together, pushed through the boundries into the physical universe and shot out what we know as the old gods. Randomly flinging them across the great dark beyond, hoping that some of them would smash into a planet containing a world-soul.
Sargeras found out about these plans when he came across a planet holding one of these titanic souls and was indeed infused by the old gods. On it were a couple of Nathrezim or Dreadlords, basking in that good old void power. Sensing their evil, Sargeras captured and ruthlessly interrogated the demons until they were broken and revealed what they had learned about the Old Gods and the intentions of the void lords. If the powers of the Void succeeded in corrupting a nascent titan, it would awaken as an unspeakably dark creature. No power in creation, not even the Pantheon, could stand against it. In time, the warped titan would consume al matter and energy in the universe, bringing every mote of existence under the void lords’will. Sargeras, the undefeated champion of the titans, knew fear for the first time.