"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:
- 5th Edition D&D confines the "Evil" it detects to Outsiders, Undead, and Holy Ground 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.