Tabris, since you mention a title, I wanted to say the thing I liked best about your query was that the angel/guardian was slowly going dark because he wasn't accepting the grace/forgiveness given to him. I love that concept. So, I'd suggest playing with that for the title.Dark Fall.
Second-chance Angel (meh, this comes off as "romance-y")
Growing Dark (sounds horror-y?)
Falling Angel ("ing" vs. "en")
Own Worst Enemy
Bah, I'm bad at this.
Sorry, I wanted to play, too.