Arthas is corrupted by Frostmourne.
Illidan goes mad in TBC.
Kael'thas goes mad in TBC.
Garrosh will be corrupted in MoP.
Deathwing is made insane by the Old Gods.
Malygos is driven insane by Deathwing.
The Eredar are corrupted by Sargeras.
Anyone else tired of the "Oh, he/she is insane/mad/corrupt and therefore evil" routine over and over again?