this "Light" and "Dark" of our is never always meant "Good" and "Evil".
both 'light' and 'dark' have their own monstrous entity, so neither "Flooded by Light" nor "Flooded by Darkness" is a good thing.
and by saying 'Flooded', the shard is pretty much became a void (13th shard, flooded by darkness). and of course, we arent gonna make that happened again.

our role in Shadowbringer is pretty much we make things go into an equilibrium again.
we push/kill the 'bad light', and bring the Darkness back into the first.

"How we became a WoD?"
In my opinion, its the same as how we became WoL.
our goals as a WoL is bring back the Light from the darkness.
and when we became WoD, we bring back the Darkness from the Light.

we have play and bring an intense amount of 'light' into the shard, so if we gonna make it into perfect balance, we also need to bring the Darkness right ?