First, "evil" must be defined more as "unholy," or "against the gods." I think that IG, this is mostly the route taken, though Harm Evil does work against certain of the Gods' constructs (I believe this was a flaw in the game's early design). This makes room for the duality of light and dark, and the balance between the too, which is what I would call "good."

The unifying principle of the Immortals is struggle. It is from the opposition of forces and the struggle between the two which balance, good itself, springs.

Each of the immortals has a positive and a negative aspect. These very aspects of the same god often, if not always, espouse contradictory, if not truely opposite, positions (painful death vs. painless death). It is in parties of both sides struggle against one another that the whole of the god, and the balance is found. So the role of someone holy is to perpetuate the balance. Some will work for the side of light, to further their struggle. Some will work for the side of dark, to ensure that there is an opposition to create the balance. Some will serve the balance itself, working for whichever side, or both, as is needed to ensure the balance is maintained. So long as they work for the Immortals, they are not "evil."

In Gheist's Ideal DragonRealms (TM), the paladin's would be allowed to serve whichever side, dark or light, they chose. Either is serving the gods, either is a holy warrior. This is how clerics currently work. I think that the problem is that paladins are based on a belief system not really supported quite accurately in DR. They hold that certain things (things the dark gods patronize) are just plain wrong. This effectively means the paladin guild is predefined as holy warriors of the light. There is nothing wrong with this in itself, but there is a big hole where there should be holy warriors of the dark gods. The lack of the compliment makes it seem as though the dark side is somewhow wrong (if the only thing which is called good is the light.....), which is not how the Immortals are set up.

-Gheist