I second the last post. Angels are good, but they are also GOOD to the point of being alien and inscrutable. Add a double twist instead of single; I will let her go? Make it clear that the enemy has a very clear honor streak, and will respect noble actions on the party's side. The choice comes down to let someone important to the PC's faction go to her death in a duel, or continue leading. But instead of the enemy respecting it, he feels mocked and sacks an important city- he just so happens to have also put down a cabal of devil-worshippers who took the initiative and were starting a coup while all the nobles were on the front lines.
Alternately, if you use a more common conception of good folks becoming angels, each promise is something that specific angel needs done in order to let go of their mortal life and get down to brass tacks. This is like a lifetime drama where someone dies, becomes an angel, can't meddle directly and so conscripts some heroes. I don't have concrete examples here, though. It was my first thought, but isn't half as good as what's already up.
Alternately, if you use a more common conception of good folks becoming angels, each promise is something that specific angel needs done in order to let go of their mortal life and get down to brass tacks. This is like a lifetime drama where someone dies, becomes an angel, can't meddle directly and so conscripts some heroes. I don't have concrete examples here, though. It was my first thought, but isn't half as good as what's already up.