I'm not wishing to be controversial here, but surely the King of Darkness is God? I mean, let's look at the facts here:
I have been assured that, at least according to many Christian doctrines, Satan was originally an archangel - created by God Himself - who then fell to become the Devil;
God, being omniscient, created Lucifer with the knowledge that he would turn evil, whereas Lucifer had no choice in the matter since he was predestined to become evil;
God, being the only truly omnipotent 'being', could cause Lucifer to not only cease to exist, but to never have existed - and with no effort. Therefore, it is by God's decree that the Devil - and, by association, evil - continues existence.
Of course, I'm assuming this idea has occured to others, and Christian Philosophers and so on will have worked out why this is wrong - after all, that's what they're there for. So I submit this merely as a hypothesis, and will update with any corrections I get /msg'd with.
Oolong says: Hard to argue with that I'd say. I suppose the get-out clause, such as it is, is that He is also King of Light. *shrug* - this points me towards a disclaimer style statement - God would only be the King of Darkness in his capacity as King of Everything, including all things pretty and fluffy and nice.