I'm kind of late to the game here, but here's how I understand the trope and the idea of joining the heroes not so much out of a Heel-Face Turn
but more out of disgust with one's own side.
I see it as sort of similar to Reformed, but Not Tamed
with a slice of My Species Doth Protest Too Much
in that the character thinks their side is "doing it wrong" as opposed to being inherently bad.
So, for example, I'd expect a Defector from Decadence
in the Star Wars verse to be someone who supported the Empire as well as the principle that it should crush rebellions, but then ended up disgusted by its cruelty, corruption, etc. That might lead them to aiding the rebels, but they would do so because they are against the Empire, not because they support the rebellion.
edited 9th Dec '13 7:32:11 PM by Hodor