I guess I'll throw something in.
If God is all powerful, all knowing and in all places at all times then it's clear that Lucifer is not an accident. Lucifer doesn't possess some magical God defying force field, does he (like I actually don't know how Christianity or Judaism or Islam explains why God doesn't just smote Lucifer or whatnot, so maybe Lucifer does have the ability to defend against God's omnipotence)? So my assumption is that evil Lucifer is part of the plan.
Plus, it just makes for a more compelling story. You always need an antagonist. They never would have sold as many Bibles as they have without one.