My whole system came about because I wanted true evil, rejection of creation, but I wanted shades of gray in religion. In my reading I know that "evil" religion don't exist in reality.
This discussion deserves an entire post, because Rob's comment raises a couple of very important questions: Where does evil come from? Can a religion be evil? And a not so important question: How is it possible to create a cosmology for a RPG world that has shades of gray with resorting to some kind of paganism/polytheism?
Let me start with the source of evil:
And the Lord God commanded the man, "You may freely eat of every tree of the garden; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall die." — Genesis 2:16-17
When looking at these verses, we must remember the definition of good and evil — God is good; evil is the absence of good/God. Thus, by eating of the fruit, Adam and Eve reject God because to know evil is to know a world without God. They tried to become like God without God.
Creation was brought forth by God from nothing:
I beseech thee, my son, look upon the heaven and the earth, and all that is therein, and consider that God made them of things that were not; and so was mankind made likewise — 2 Macabees 7:28
Thus, when Adam and Eve chose to turn away from God and know a world without God, they chose nothingness — their choice introduced death into creation. As priests and caretakers of God's creation, they dragged all of creation with them towards the nothingness that everything came from. Sans God, it is to nothing that we shall all return. The word adam means humanity. So, the source of sin and evil in the world is us.
The tragic flaw of the pagan world-view is that it abdicates human responsibility for virtually every aspect of life. War doesn't exist because Ares invented it. War is our creation. We are responsible for it. We are the source of evil through our separation from God.
Insofar as a religion encourages or requires behavior that separates humanity from God, it can be evil (any way you slice it, human sacrifice isn't good). However, every religion can have shades of truth — it can correctly understand an aspect of God, but fail to accept the fullness of who God is. The problem is, there are very real consequences that come with these failures.
In order to get shades of grey into the religion of an RPG world, one really doesn't have to look much further than a dogmatic history of Christianity. For purposes of illustration and inspiration, let me walk through some heresies from people who understood themselves to be Christian, and the consequences of their belief:
- Gnosticism: There are several variations on a theme, but Gnosticism basically boils down to a rejection of matter as the creation of an evil demi-urge (i.e. the OT God). Salvation comes through the knowledge of the true God, which is the light/soul trapped inside a fleshly prison. As a consequence, things like murder, hedonism, extreme asceticism, and abuse are all acceptable because all matter and flesh are evil. What we do with it has no bearing on our salvation; only knowledge does. In terms of D&D, Gnosticism is nicely expressed in the explanation of Chaotic Neutral offered by the 1st ed DM's Guide.
- Arianism: Arius and his adherents insisted that Christ was a creature — he was part of creation and did not share in the Father's essence. This reduces Christian eschatology (the experience of the Kingdom of Heaven) and ontology (the nature of being) to a moral/ethical system (and one that is impossible to live up to). With no eschatological or ontological justification, this moral/ethical code is doomed to fail, since everyone sins. As such, the only way to justify and enforce this moral/ethical code is through coercion.
- Nestorianism: Nestorius and his ilk held that the human and divine natures of Christ were separate persons conjoined in the man Jesus of Nazareth. Such a reality justifies a compartmentalization of human activity. Our religious lives can be separate from our daily lives. Thus, a man can justify being a pious and loving husband and father at home at the same time he is a torturer and killer at work without any conflict between these two aspects of his life.
- Monophysitism: This heresy held that the human nature of Jesus was absorbed into the divine nature of Christ, leaving Him with one nature. Overemphasizing the divine nature of Christ devalues human nature, and thus humanity. When humanity gets devalued, it becomes easy to justify things like slavery, racism, genocide, etc. because the definition of what it means to be human can be narrowed to fit whatever category you need. Thus, Group A is human and Group B is not because they don't have what Group A does. Enslaving, discriminating against, and even killing Group B is justifiable because they are less than human.
Thus, without ever having to resort to a pagan cosmology, there are plenty of ways we humans have figured out to impose shades of grey onto Christianity by rejecting certain aspects of God.
I have been accused by players of being the most frightening Referee they've ever played with. The reason being that I apply my understanding of evil to my worlds. I insist on a clear dichotomy between Law and Chaos, which lends itself very well to the illusion that everything is black and white. In reality, evil corrupts everything and the most terrifying monsters in D&D are human. Yes, my monsters are physical manifestations of sin, but the true evil in my worlds originates with people, not monsters. As such, dealing with the folks back home can sometimes be more dangerous than delving in a dungeon. Survival rests upon my players' ability to recognize sin for what it is. Monsters, as personifications of sin, help us to do exactly that.