Excellent idea. This would work really well if he came back a bit insane from his time on Mercury. Then his crusade against the Fallen could be more about revenge and anger than trying to save Earth. Maybe have him go too far during the fighting and he falls to the Darkness, something like that.
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Yeah, maybe Something like Harvey Dent from The Dark Knight, where we have kill him and keep his fall from grace a secret. If the City found out what has become of their hero, their moral would be devastated. "Die a hero... or live long enough to become the villain" It could go like that or he would really be a pure and just hero that leaves/dies after his work is done.