Detect Evil

"At will, a paladin can use detect evil, as the spell."
"...If you are of good alignment, and the strongest evil aura’s power is overwhelming (see chart), and the HD or level of the aura’s source is at least twice your character level, you are stunned for 1 round and the spell ends."

The obvious problem with the above is that Detect Evil is usually used for terrible purposes by a lot of not very bright players and DMs (who try to use it as a "detect people who have committed crimes I am willing to kill people for committing" spell). Further, it has a bad habit of imposing an objective moral system on the setting, which is frequently thought a bad thing in these more "enlightened" times, although really the main excuse for that is that writing an air-tight setting of objective morality is very hard and beyond the abilities of most authors, or whatever hack is posing as one this edition.

Various methods used to correct these flaws at a system level include:

• In 5th Edition D&D the spell is Detect Evil and Good. It confines the "Evil" it detects to Outsiders (such as aberrations, fiends, elementals, etc.), Undead, and desecrated grounds or items of any sort. Human/Orcish/Elvish/etc. evil not detected unless they've been working directly for the Supernatural.
• Embrace it, and then "deconstruct" it into uselessness. Didn't donate to the orphanage? That's as Evil an act as human sacrifice to the Nine Hells!
• Make it go off on innocents due to curses and so on. Forex, in Werewolf: The Apocalypse, people who have eaten at a Pentex-owned fast food chain smell as Wyrm tainted for a while afterwards.
• Make it entirely, explicitly dependent on the opinion of the God whose Detect Evil you're using. Doesn't quite solve the "detect people I should murder" problem at face value (depending on the Gods in question), but it does dilute the "objective morality" problem quite a bit.

For DMs in need of gimping the "looking for targets I can murderhobo freely" problem:

• The DM should note that casting Detect Evil is incredibly rude--a bit like sniffing the clothes of anybody you come in contact with.
• This is an obvious spell use, even for Paladins. Given that you're probably playing in a system (D&D 3rd or Pathfinder) where meta-magic feats are a thing, have people react as if this is an attempted attack.
• Have important, dangerous people trigger it. E.g., The Lawful Evil head of the Town Guard who takes bribes, but is otherwise the best man for the job.
• The "Protection From Evil" spell explicitly states it will abjure Outsiders ("enchanted, conjured or summoned" in 1st edition), even neutral- or good-aligned ones you would want as allies.

Never lie to players; if they are using Detect Evil as part of an investigation, feeding them wrong information is just petty. But if you really got to get out of a corner:

• "Nystul's Magic Aura" is a first-level arcane spell that lasts 1 day per level, since 1st edition. In 3rd edition, "Misdirection" is second-level and lasts 1 hour per level. Now if only the heroes thought to detect for illusions instead of evil...
• "ah, you're detecting the cursed armour I'm wearing. Can't take it off, you know what cursed items are like."
• The halfling of indeterminate alignment carries around a sheet of lead with fast reflexes, "for cultural reasons."

And finally, just talk to the player. Explain how the behavior is disruptive. People play games to have fun. The main purpose of paladins having access to Detect Evil is so that they don't blow their Smites on immune targets. Start there and work up.

The Moral Dilemna (or, Why Paladins Can't Actually Just Slaughter All Evil As Soon As They Detect It)

Let's say your paladin uses detect evil and realizes half the seedy tavern the PCs are in is some lesser degrees of evil, along with maybe a few "darker" spots from those who are tapping more directly into dark powers. If you think your paladin should start drawing a sword and mowing down folks, you just broke your alignment as well as your paladin code, and it's gonna take a thick rug to pray for that atonement later. Why?

Because if you kill evil ordinary people, you just fucking helped the forces of supernatural evil in the cosmos, you fuckwit. No, really, stop a moment and think. Where do evil souls go where they die? Straight to the Lower Planes... to become even more stuff and/or minions for evil deities, fiends, and others to use. Asmodeus thanks you for your service.

"But, but.. how am I supposed to do good if I can't just kill evil things all the time?"

You redeem evil people, genius. Use diplomacy to get them friendly to you, then start changing their behavior in subtle ways. Get them to help turn others, if not towards goodness, at least away from evil so that you deprive your actual enemies - the denizens of the Lower Planes who feed and grow powerful on the evil of ordinary people. More than that, every good soul you save who makes it to the Upper Planes? You just recruited them to the cause. Some may take a while to climb the ranks, but that's why it's important to save as many souls as you can.

Keep in mind, fiends and most undead are totally legit kills. Fiends are built of evil souls who already made their permanent choice of affiliation in the cosmos; they chose Team Evil, and they have to accept the consequences of that choice (i.e. getting cut down by that holy avenger sword). Sure, a few might be redeemable, and that's a worthwhile challenge, but for the most part you needn't be concerned with attacking a fiend. The trick is, detect evil can't penetrate a disguise, so you have to stalk a bit and figure out why the town blacksmith has an aura like that of a pit fiend. And killing the undead is generally considered the tactic of "kill them all and let the gods know their own", since a lot of them get made against their will; they only get Neutral Evil alignment because they are given that unnatural urge stuff like eating flesh, drinking blood, draining life energy, etc. However, even if it looks rotting, you need to pop detect evil anyway, just to be absolutely sure. It might be one of the few non-evil undead that do actually exist out there, or it might even be a deathless being (who are similar, only infused with a burst of positive energy instead of negative energy). It could also just be a really gross looking peasant. Better safe than sorry, anyway.

DMs reading this should absolutely give their paladin players these opportunities when the party gets some downtime between adventures. Remember; paladins do not really covet wealth. So long as they save souls and stay Lawful Good, their big reward in the Upper Planes is already there waiting on them. The more souls they save, the more friends they'll have up there, and probably get even better promotions for it (just as devils get them for damning more souls).

The Other Side of the Coin: The obvious counterargument

Redeeming the degenerate and base servants of Evil is all Noblebright and good, but consider the parable of the soiled city upon the hill that was the city of Sodom and it's twin sister-city Gomorrah. Sodom was a wicked and perverse city inhabited largely by ordinary people, who were mundane in their wickedness for the most part; with the notable exception of the greater sins of their society; and they could have perhaps been delivered from evil and redeemed if they truly desired it.

So why did they not? And why did God destroy it?

God destroyed the cities not only because of their wickedness and refusal to repent, but also because their evil would have spread and because none of the evils that grew in Sodom would have swollen to such legendary heights if the normal citizens had just refused to tolerate the open practice of such wickedness. It was only by their collective apathy and tolerance for wickedness that the city sunk to such lows, as the greater acts of evil would not have been possible without such a tolerant and permissive society to fester in, and thus it was by the works of the common man of the city that it was destroyed, 'The only thing necessary for the triumph of evil is for good men to do nothing' taken to its logical extreme.

As Gygax was indeed a faithful man who knew his Bible, Evil does indeed have ways to spread and grow entrenched when permitted to do so in a society. Allowing Evil to become entrenched will make it much harder to dislodge, as the big kids down in the Lower Planes will take notice and provide aid to advance their agenda within the mortal realms. Thus societies (and by extension the people that populate them) that tolerate the presence of Evil make the war against it all the harder for the side of Good. As such, Paladins and their Orders should continually seek to conquer or destroy such societies, as they are nothing more than demon-manufacturing-plants.

"But wait, what about the evil dead becoming demons in the hells?"

What ABOUT them? Evil's agenda within the mortal realms is to corrupt souls into ever-increasing depths of depravity and wickedness; as the more wicked and evil a soul, the easier it is to make a more powerful demon out of them when they go to hell, especially fiendish and wicked mortals often skip several steps of Hell's tortures/training when they arrive in their hell, rapidly becoming a powerful demon of renown. As mentioned in the previous section, you should try and avoid this altogether by redeeming the nasty bastard, but such mercy needs to be ASKED for, as repentance is not something passively done, but rather actively done. When a regular plain old evil-doer gets sent to his rightful reward, they often become the lowest type of demon on the totem-pole, as they're not an especially vile or wicked old soul steeped in Evil who gets to skip several steps and become an uber-demon right away, thus what Hell and the other demons down there have to do is "break them in", and these tortures and training and whatever vile indignities that are done to show that damned soul his place in the infernal hierarchy take time to do so, a looooooong time as a matter of fact.

This is in fact quite good for Team Good, the operative word here being time. As the time that that damned waste of space is suffering through in the recruitment of Hell is time that they are not alive, spreading the taint of Evil ever further. It is time that they are not alive, matriculating into a more vile and wicked soul that is ripe for conversion into demon-hood. It is time that they are not spending bring OTHERS down to their level, whether it be through their own petty cruelties, their permissiveness towards the truly evil among them permitting their suffering at their hands, or their apathy towards the suffering of others that drives them into committing evil acts just to stay alive. It is time that they are not spending supporting the awful society which birthed them, thus making it easier for Team Good to destroy their debased country and weaken Evil's foothold in the world.

If you should take anything away from this, it's that good Paladins should not tolerate Evil being within their presence, if they're not going to smite it right away, then they should either engage in some good old pseudo-Christian shouting and rebuke and chastise them, or not be near Evil at all unless they're removing it with force. This can be quite disruptive, which while perfectly justified in-lore, can be a bit detrimental towards actual gameplay. Consider having the Paladin's Order be choosy about where they are sent, or mandate that their holy warriors maintain some sense of subtlety when abroad in foreign locales, after all, as Paladins are essentially the foot-soldiers of Good, the forces of Evil would likely not be pleased to see even a non-hostile Paladin amongst them.

Of course, the oft quoted "whoever fights monsters should see to it that in the process he does not become a monster" comes to mind here, as such a merciless attitude can hardly be considered as appropriate for a Lawful Good character (or a purely Good character of any stripe for that matter), but that can be considered another limitation of the traditional Lawful/Chaotic-Good/Evil Alignment system. Not to mention that real-world philosophies of ethics constantly debate over these points through the schools of Consequentialism and Deontology. At worst, such a character may devolve into an especially weird form of Lawful Evil that is somehow aligned against Evil, but realistically, such a character would either end up outright defecting or dying alone without support from their former Good patrons when they are exiled for their transgressions against moral decency. Mind you this is something to keep in mind only for the pure Edgelords of Law, these points in moderation can do wonders to alleviate the problems of Stupid Good.

TLDR; your god isn't going to sweat over Evil getting a few extra Manes/Lemures in the long run, don't worry about being knee-deep in the dead, at least not until you're functionally indistinguishable from them.

Law for the Law god! Justice for the Throne of Justice! Let the multiverse BURN!

See Also

• Always Chaotic Evil for a closely related problem
• Detect Evil Storytime
• Powder Keg of Justice
• Here's some possible solutions for DMs in D&D
• Excerpt_from_"Life_and_Times_of_Andros_Fairheart"