Not entirely original but I have a few adapted ideas.

1. A Celestial that has incredibly high standards that finds either the entire world, or maybe a region wanting. Perhaps he was dormant for a time and after being revived is aghast at what he sees as the decay of society. At first he may make targeted strikes hoping to save/convert the population but after a while he grows frustrated and goes for the 'kill them all and start over' theory.

For this I starting thinking about someone coming from out of time and seeing modern society. Think of a 'proper' Victorian gentleman that was dropped into the 21st century and then made to watch Jersey Shore and The Real Housewives of whatever. Now give that guy nearly God-like power and a mission save mortals from themselves.

1.5 One side concept to this could be a 'righteous' Celestial that works to destroy an 'evil' nation. Well the king may be evil, the nobility evil, the church evil, etc but the people are just people. Sure some folks are evil too, but a lot are just trying to survive and a good number may even be good. The PC's may have to decide between fighting the Celestial to protect the 'people' or trying to overthrow the evil regime to ward off divine destruction.

For a modern analogy think of Iran or North Korea. Sure Kim Jong-un might be a nutbag, his government a totalitarian nightmare, and the army paranoid, but there are 24 million people who are just unlucky enough to be born north of the DMZ. If NK were to nuke Tokyo for some stupid reason, the American nuclear response could be analogous to divine wrath.

2. A 'long game' Celestial that supports various evil causes in order to keep various threats fighting each other rather than anyone else. IE if your setting has the traditional devil vs demon Blood War, then maybe this angel is helping one side or another to maintain the balance.

This could evolve to where the Celestial starts to enjoy the 'perks' of being around evil and finds some way to mentally justify that he's still helping the greater good. IE he lives a decadent lifestyle with booze, women, gambling, etc to keep his cover and work against a greater evil. Who cares about hookers and blow when the alternative is the baby eating third cousin of Cthulhu.