It all depends on what you, personally, have been fighting for and to whom you owe your allegiance. Personally, I don't have a problem with taking up the side of darkness as long as it's to save people. "Warrior of Light" is just a title, and my allegiance is ultimately to my own sense of right and wrong, not Hydaelyn.
Narrative wise? The only thing I could think of is that something happens that tips the balance too far to Light, causing a Flood of Light like on the first Shard and necessitating taking up the power of Darkness. How we could use the power of Darkness and still oppose the Ascians... well, that much I couldn't guess at beyond noting that the Ascians are more aligned with chaos, while we could be taking a more moderate path and seek freedom from the tyranny Light can impose at its extreme.
Without further details (who the voice belonged to and why it was able to rip the Scions' souls from their bodies) anything's game, really.