Spawn doesn't require Darkness, he CAN feed off Darkness.
Spawn drawing power from darkness, just means that Darkness can weaken him.
Similarly to how a person who can control sunlight, would be able to weaken Superman.
In his universe, Darkness is nigh-omnipotent. As long as it is dark, there is pretty much nothing that he cannot do, likewise for his counterpart the Angelus, when it's light.
The Darkness, was the primordial power before creation, then God let in light and the Angelus, which sparked the creation of the universe/multiverse.
The only version of Spawn that would actually defeat the Darkness, is the one empowered by The Mother, that kicked around God and the Devil.
He can also create intense light to weaken Darkness extremly, he was heavily stunned by burning phosphorus, Spawn can amplify that a couple times easily.