Scruffy, what you describe could very well be what this song is about, but it is difficult to tell because the lyrics do not provide details on the nature of the two “alters” she seems to be dealing with.
But, more generally speaking...
The “good girl gone bad” archetype is quite common in popular music and one can cite plenty of examples: Rihanna, Beyoncé, Britney Spears, Christina Aguilera... Sometimes the good/bad states do coexist from the beginning and the ambiguity is what makes it work. Whether this good/bad recurring pattern is genuine, orchestrated or even forced upon the singers is difficult to determine. Many times, the singer will pretend to have several personalities, to have a good and a bad side to their soul, to be “possessed” while on stage, to lose control while performing certain songs or while being a certain persona. Is all this just for publicity, is it just to reach to new fans, is it a natural extension of reality (no one is one-dimensional and we all have good and bad sides) or is it just that the singers are controlled or programmed by an external force? Myself, I think you might have all of these playing together, but in many cases, I would say that the individual (the singer) is simply controlled and manipulated by others. And yes, I beleive this can and does go to the extreme cases where you enter the realm of mind control and programmed individuals.
The good/bad duality can be conveyed in many ways; attitude, lyrics, imagery and symbolism can all be used to express this concept. There is also plenty of play on contradictions and inversions of states or concepts: good is bad, bad is good, Satan is God, God is Satan, up is down, down is up... Gainsbourg once said of VP: “Paradis, c’est l’enfer” (literally, “paradise is hell”).
Another example, see what Alizée is wearing when she performs this song at the Tel Aviv Gainsbourg “Concert Hommage”:
This screams of good/bad (white/black) duality.