The Greek word for Devil is "diabolos," which means “slanderer” or even more literally, “the one who divides.” The Fall is division. When humanity turned its back on God and tried to be divine without God by partaking of the fruit of the tree of knowledge of good and evil, humanity divided itself from God. Since humanity was given the special role to keep the garden and all of creation for God, humanity took all of creation with it in the Fall. Creation, in the person of Adam (which literally means “humanity”), tried to exist without God. Since creation came from nothing (and will, by nature, return to nothing), death, decay, destruction and sin all came to rule over everything. As a sign of this, Cain murdered his brother Abel — humanity is even divided against itself.
Unity in Diversity
The Planar Cosmology of D&D, especially that depicted in the Planescape setting, simulates fallen creation extremely well. All of creation is divided against itself, warring over philosophical absolutes, where even those who profess to be Good not only war against others who claim to be Good, but will ally themselves with those who are Evil to battle a different variation of Good. Power is all consuming, where beings from every Plane scramble to become gods, without God. In a world where magic exists, this illusion of divinity without God becomes even more powerful and difficult to see through. Belief has the power to shape the multiverse, and everyone strives to bring some kind of unity through belief — a desperate attempt to be God without God. Of course, all such attempts will fail and only bring about more death, destruction and sin. The only being able to save, to bring true unity in the vast diversity of the multiverse is God. This unity is made manifest in the person of Jesus Christ, where all of humanity, in its infinite diversity can become one. This radical equality happens not by eradicating diversity, but through that diversity. Each individual brings to the Church — the Body of Christ — her own unique and unrepeatable person, talents and skills. Thus, with the vast diversity of the D&D multiverse, the Church represents the one truly divine means of unification, despite the radical differences that exist across the planes.