Gamebreakers: "You know how plots go: They are always using one device or another to move themselves along. Parents die to fuel revenge. Inner monologues give us insight into a troubled character. Enemies become friends, and friends become enemies. This list looks at the latter. Any way you slice it, whether they go from friend to foe or vice-versa, they are betraying someone. Here are the Top Ten people who turned their cloaks, for good or for ill."
(Major game spoilers ahead, particularly for the recently-released Portal 2)