It IS interesting, isn't? That a race of servants could be capable of falling at all? Perhaps there's more going on then meets the eye - perhaps the White God does, indeed, have a Plan at work, say to breed a champion not of Heaven or Hell but of humanity, to shield them in the coming age - the one where Downbelow and Heaven finally start scrapping for keeps. Perhaps this champion has no relation to the White God's endless war for dominion over mortal souls, but is instead a contingency plan, a shield to be set against the worst - Downbelow's victory, an incursion from Outside, or some unforeseen upheaval where, say, Faerie decides that ganking God is in its best interests.
So, take an angel, a being of perfect virtue, and tear something in it; leave a weak spot, an open wound into which corruption can slither, driving it to Fall with the rest already corrupted (as to how that happens, who knows? Maybe there's an even bigger Plan. Or maybe the Morningstar WASN'T an angel - maybe it was an Outsider with the same thematics as the White God, cast down and inverted, seriously pissed off and looking to tear down everything the big guy wants to build up). Let it drown in darkness, see the worst of everything, slide its essence into a tarnished silver coin so that it can make war against everything it was made to serve and defend.
But leave a secret in its heart: the power to heal that torn spiritual ligament, even as warped and stretched as it has become, just as mortal muscle strengthens. All it needs is the faith to restore it, stronger and better then before - but of course, faith is not something the Fallen have much access to. So let it wander from host to host, reaping death and suffering, always feeling faintly that something is missing - until it finds a bearer unlike any before, one with the Conviction and Discipline to stand against its vast evil, to shine a light through that obfuscating shadow, right to the forgotten core of it.
It'll be a harrowing journey from there to redemption, but what's a PC without angst and long background arcs?