Apparently, a double agent CAN be the source of an epic debate (Metal Gear Solid)
Note: Ocelot wasn't a villian, in the conventional sense. <obvious spoilers>He pretended to work for Liquid Snake, but only to get the Optical Disc. He then worked for Solidus Snake, to keep an eye on him. In MGS4, he was never possessed by Liquid, this was just a ruse to fool the Patriots. He was a follower of Big Boss, so he was not a villain, at least not since MGS3.</obvious spoilers>