A character who joins either the protagonists or antagonists— not so much because he agrees with all their ideas as because he can no longer tolerate the opinions or actions of his old group.
This is usually because their methods are too extreme, ineffectual or illogical.
Another major example is when the qualities he admired in his allies are lost by them.
If the character is defecting from the antagonists (or at least from a group whose goals or methods are less than admirable), perhaps because they are a Guilt-Ridden Accomplice, expect the former cronies to become All of the Other Reindeer.
