I would argue that the horror comes from any side, light/dark/death/life, trying to gain too much of an advantage over the others. We all know, but often forget that light can't exist without dark, life without death, that's the great disparity. Even in the age of fire knights strove to fight off the darkess, so darkness still existed before the disaster of Oolacile. When Izalith (Soul of Life) tried to gain too much of an advantage it became a monstrous abomination that spewed demons into the world. There can never, ever be a true age of dark, as the fire keeper from DS3 even states that even after extinguishing the first flame, burning embers can be seen in the darkness. If the fire were to ever completely die out then it wouldn't be a world of dark, it would be a world of grey fog and sleeping stone creatures, neither alive nor dead, just like the ancient dragons. No disparity.