Is it wrong for NPCs to block a 'detect evil' check by a PC?

[TheEvil]

Here are the things I keep in mind to mitigate this problem:

First, it takes three full rounds of concentration to locate the origin of the evil. Ever stare at somebody for 18 seconds straight? They'll know that you're up to something.

Secondly, casting spells or using abilities like this without someone's permission is likely to get you bonked on the head. That fighter there doesn't have any points in spellcraft; you could be detecting evil, or you could be casting magic missile. Safer just to bonk you first. After all, you were casting a spell on him; it's self-defense. Or so he'll claim when it comes up in court.

Thirdly, evil doesn't mean guilty of horrible crimes. That guy who radiates evil could be a petty thief, or he could be a murderer. The strength of the aura in the spell comes from the power of the person, not the intensity of the evil. A 11th level person who is evil, but has not committed any particular crime recently (or at all) radiates more strongly than the 3rd level guy who just killed his entire family.

Also don't forget the oft overlooked aspect of a Paladin's detect evil: it is a spell-like ability. He still has to go through all the casting motions.

 

[Griffith Dragonlake]

Lawful alignment doesn't mean law-abiding.

According to the SRD "Lawful characters tell the truth, keep their word, respect authority, honor tradition, and judge those who fall short of their duties. … "'Law' implies honor, trustworthiness, obedience to authority, and reliability." See http://www.d20srd.org/srd/description.htm#lawVsChaos

Numion please explain why you believe that "Lawful alignment doesn't mean law-abiding." I'm having trouble wrapping my head around your assertion.

 

[shilsen]

Also don't forget the oft overlooked aspect of a Paladin's detect evil: it is a spell-like ability. He still has to go through all the casting motions.

It's not oft overlooked. It's actually people using Spell-like abilities as written. They have no verbal, somatic or material components. PHB, pg.180.