During his never ending war against the evil in the universe, Sargeras became increasingly depressed by the chaos he saw wrought by all kinds of evil. While his confusion and misery deepened, Sargeras was forced to contend with another group intent on disrupting the titans' order: the Nathrezim. This dark race of vampiric demons (also known as dreadlords) conquered a number of populated worlds by possessing their inhabitants and turning them to the shadow. The nefarious, scheming dreadlords turned whole nations against one another by manipulating them into blind hatred and mistrust. Sargeras defeated the nathrezim easily, but their corruption affected him deeply.
Shaken by the evil of demons in general and the nathrezim in particular, Sargeras began to despair of his task and gradually slipped into a brooding depression. As doubt and despair overwhelmed Sargeras's senses, he lost all faith not only in his mission, but also in the titans' vision of an ordered universe. Eventually, he came to believe that the concept of order itself was folly, and that chaos and depravity were the only absolutes within the dark, lonely universe. His fellow titans tried to make Sargeras realize his erroneous way of thinking and calm his raging emotions, but he disregarded their more optimistic beliefs as self-serving delusions. Storming from their ranks forever, Sargeras set out to find his own place in the universe. Although the Pantheon was sorrowful at his departure, the titans could never have predicted just how far their lost brother would go
From wowwiki: In the original lore, Sargeras was sent to confront the insidious eredar prior to his confrontation with the Nathrezim. This has now been retconned by new lore that establishes that the eredar, and other races were transformed into demons by Sargeras. AAAAND from Wowpedia: In the original lore, Sargeras was sent to confront the insidious eredar prior to his confrontation with the Nathrezim. This has now been retconned by new lore that establishes that the eredar, and many (but not all) races were transformed into demons by Sargeras. It was fighting the Nathrezim, Ered'ruin, Sayaad, and Infernals that corrupted him as revealed in the second issue of the WoW magazine.
He might not be corrupted by Tich himself, but he surely hasn't been corrupted by the Eredar. It feels good to prove people wrong.
