1. Deathwing, the Fallen Dragon
World of Warcraft is full of treacherous lords, scheming mages, and just a whole lot of generally conniving bastards. Sometimes, though, a story on this scale needs to get classical. Sometimes all that's called for is a one great, big dragon to tear shit up the old-fashioned way.
That was Deathwing the Destroyer. In World of Warcraft's Cataclysm expansion, his return to the world at large was literally earth-shattering. This reunion tour altered the very landscape of Azeroth, the world on which war is crafted, which players had grown accustomed to over the course of five real-world years. Then he got himself thrown into the sea, and that was the end of that.
Long before Cataclysm, however -- even before World of Warcraft itself -- he was a major player. Also known as Neltharion the Earth-Warder to his friends (back when he had friends), Deathwing was once a pretty swell dude. He started as one of five super-dragons empowered by the Titans (the gods of the Warcraft universe) to help shape the world. Deathwing created the very mountains and rivers that he later blew up. In retrospect, that doesn't seem all that bad when you consider they were his creations in the first place.
As these things tend to go in all Blizzard games, not just Warcraft, Deathwing became corrupted. Unlike most big-leaguers in this universe it took more than some two-bit demon to turn the Earth-Warder into the Destroyer. Rather, it was the Old Gods, a bunch of knockoff Lovecraftian space monsters, that did him in. When lurking horrors whispering from beyond the cosmos are involved, it's hard to fault the guy for succumbing to his more Arthurian tendencies.
2. Kael'thas Sunstrider, the Mystical Junkie
Of all the villains on this list, Kael'thas might be the most tragic. Not because he's the most despicable or the most powerful, but because his heel turn stemmed from a real-world problem: addiction. Granted, it was a magical addiction. But hey, those marijuana PSAs in the 90s that told you smoking the dope would cause you to spontaneously combust weren't all based in fact, either.
Before getting addicted to that sweet, sweet pixie dust, Kael was about as much of a goody-two-shoes as you could expect. He was every bit an elf in the Tolkien tradition. That is to say brave, altruistic, and Aryan in the way only a racist Englishman from the 50s can really appreciate.
Things went bad when it became clear that Kael and his people were fatally reliant on the magic of their Sunwell -- a great, big pool of mana juice. When that went kaput, the Blood Elves (Kael and his crew) were born. Even then they remained on the side of the angels. It wasn't until a racist human Marshall named Garithos used their consorting with other magic addicts, the Naga, as an excuse to lock them up that Kael started looking for other allies. He chose Illidan, himself an elf traitor, as a solution to his magical cravings.
It didn't last. After watching Illidan get his ass kicked during the Burning Crusade expansion, Kael'thas joined up with the demons of the Burning Legion. Presumably it seemed like a good plan at the time. Unfortunately for him, though, that alliance wound up with him getting repeatedly beheaded by the nameless hordes of WoW players around the world. Remember, kids: Stay away from the hard stuff.