The opposite of the Warrior of Light is not the Warrior of Darkness, but an Ascian. Both the Warriors of Light and Darkness were blessed by Hydaelyn and follow her ideals ultimately. Even when Albert and Co. became WoDs, their goal was to save the aether (life) of their Shard from nonexistance. I can easily see us becoming a WoD not because we havr to fight against Hydaelyn, but because Hydaelyn herself says we have to do that for some reason. We know she cares about preserving life, so I could see her be willing to do whatever she has to to preseve it. Even if that means asking us to spread Darkness.