It's always fun learning that you're a total failure.
After David's bizarre fall from grace, he has absolutely no idea what to do. No one in known history had ever been sent back to mortality as punishment before, and it's no surprise when David falls into a deep depression. How could a mortal fulfill the duties of a guardian angel? And how could he ever live down the shame? With no other choice, David is forced to push onward with faith, just like any other mortal, and find new purpose in life.
Yet with angels being captured left and right, and evil infecting the world, how can he just stand by and leave the fighting to the angels? Is there still a place for him in the demon hunters? Or is he doomed to watch from the sidelines as demons pick off his friends one by one?