Gul'dan
He manipulated his people into demonic corruption, got them to commit (attempted)genocide against the Draenei, took over his people by setting up a puppet as the "leader", opened a portal to Azeroth--starting a chain of events that leads to every war in future games--in a play for god-like power. During the war, he takes the clans loyal to him the night before the Horde would have conquered Lordaeron and beaten the Alliance and moves towards the ancient tomb where that power is located. He repeatedly screws over his own race--assinating leaders that could redeem the orcs, tricking them into enslaving themselves to the Burning Legion, stealing half the army right before a big battle.
In the novels, flashbacks, and other forms of retconing the older stories, you may have noticed orc antagonists becoming more sympathetic--Orgrim Doomhammer is arguably the best warchief the Horde's ever had, Ner'zhul didn't truly go evil until he was captured by Kil'Jaeden, ect. Gul'dan is the only one who was retconed into being even more evil...
