The morality of the world is not black-and-white, no matter how much people try to paint it otherwise. There are lots of shades of grey in there, which becomes a problem when dealing with D&D’s alignment system. If a detect evil spell says you’re evil, then you are evil, and have been so condemned by the very morality of the world itself. This would not be a problem if there was no such thing as detect evil; unfortunately, there is, and the paladin and the barbarian/cleric in the party both have it.
How do you make detect evil work in a world of grey morality and uncertain allegiances? The answer, to me at least, is a simple one. A paladin or a cleric’s power stems from their god; therefore, so does the detect evil spell. Detect evil returns as evil everything that your deity hates. This leads to some interesting situations where the paladin’s detect evil tells him an NPC is evil, but the cleric’s doesn’t–things like that. A lot can be told about a person if you know what gods like them and what gods don’t.
All detect evil spells will return as evil truly irredeemable characters, such as serial killers and demons and the like, unless the god is extremely depraved.
The following is a list of what will return as evil when the detect spell is cast, using a list of gods that are known to exist in my D&D world. There are probably more gods than this lurking around in the metaphysical corners of the room, but they haven’t come into the spotlight yet so I don’t know about them.
Imperial Mother Church
Jojo McMotherfucker (aka the Jujhar, Sword-God, Moon-God, the Lion): Those who prey on the weak, those who betray trust, those who are weak and make no attempt to better themselves, all snakes and snake-related creatures.
Lack (the God of Laughing Knives, the Balanced Coin, the Charitable One): Those who kill for their own gain, those who enforce the letter of the law rather than the spirit, those who hoard wealth jealously, those who destroy beauty, those who enslave, children under the age of six.
The Spice God (the Sun, the Golden/Gleaming One, the Harvest King/Queen (depending on region)): Those who destroy food, those who resist positive change, those who stunt growth, locusts.
Our Lady of the Underground (Our Lady of the Upside-Down, Our Lady of Ways, Our Lady of Means): Those who try to prevent a good time, those who are needlessly cruel, those who get caught.
Avem-Sernad (the Twin-Faced God, Life/Death, the Gaunt): Those who try to cheat death and those who hold life in no regard. Note: Avem-Sernad exists in both the Imperial and Risen pantheons.
The Risen Gods
Orsag (the Guardian, One-Eye, the Rage and the Thunder): Those who prey on the weak, those who assault the works of men, most elves.
Taborin (the Uncaring Judge): Those who break the law.
Dominarin (the Scholar, the Unbound God): Those who keep knowledge secret, those who destroy knowledge, those who burn books of any kind, most elves.
The Patchwork Man: Those who impede his travels.
Avem-Sernad: See above.
The Frozen Gods
Drowning-In-Snow: Those who try to escape the inevitable, those who exploit others for their own comfort.
Blood-In-Black-Water: Those who kill other intelligent beings and eat them. Nothing else will return as evil, including non-cannibalistic demons.
Frost-On-Flesh: Those who burn others, or take enjoyment from fire.
Heat-Of-The-Dead: Those who leave others to die, those who try to cheat death, those who should not be alive.
The Forger (He of the Flame, He of the Burning Earth): Those who destroy instead of build.
Old Jack (the God in the Mountain, the Worldbreaker, the Doom of Empires): Old Jack is not actually a god. Anyone casting detect evil using power they are claiming to draw from him is either lying or duped.