"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.
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).