I remember a villain to be who never got the chance to really live up to his evil. In fact, this happened twice.
The plan was initially developed by a friend of mine in 2e, and then tried again later in a D&D/CoC d20 Cross I tried.
The guy's name was Luminaire, and at the start of the campaign, he was a paladin hero belonging to the same organization the PCs belonged to. he was modeled, at least in part, on Sephiroth from Final Fantasy VII, apparently. He'd start off as this guy that everyone worshipped, who was pure heroism, and who was known for his hatred of evil. Then, in his pursuit of destroying evil, he'd more or less go evil himself, and pit himself against the PCs. He was known for a magical sword (a katana in a european setting) that could create a Prismatic Wall, and some other light-themed powers.
In my friend's game, he never really caught traction because the group didn't trust him right away, simply because some of the elements of the character (such as the name of his sword, which was apparently the same name of Sephiroth's sword) were known by the rest of the group - and so destroyed the sense of betrayal the GM was building toward.
I revived the concept with a different group about five years later, in a game that was a D&D meets the X-Files sort of game that is the closest I've come to a low-magic D&D since 3E came out. Luminaire was a knightly paladin investigating the mythos on his own, and the PCs would hear snippets of information this holy knight had discovered. Of course, he went a little batty due to mythos exposure, and my plan was to have him start hunting an entity that could switch bodies.
The plan was to have the PCs investigate why Luminaire had gone rogue and follow his trail - assassinations, holy ritual murders, and other "ends justify the means" as Luminaire chased this demon. Eventually, they'd follow him to an Orphanage on the outskirts of Illiandoria, and realize that Luminaire had locked every damned window in the building so no one could escape, and then lit the thing on fire.
Unfortunately, the campaign imploded before I could get there, simply because one of my players was a newbie and really wasn't ready for an investigation-style campaign; we wound up playing an Iron Heroes campaign instead.
