takyris said:With the "Depends on the Game" caveat...
Going by the book, you only get an Evil Aura if you:
a) Are an evil person,
<snip>
b) You worship an evil deity -- see "a"
So, based on the rules as written, if you see something that detects as evil, it's actually genuinely evil.
SRD said:Code:Creature/Object Faint Moderate Strong Overwhelming Evil creature1 (HD) =<10 11–25 26–50 51 or higher Undead (HD) =<2 3–8 9–20 21 or higher Evil outsider (HD) =<1 2–4 5–10 11 or higher Cleric of an evil 1 2–4 5–10 11 or higher deity2 (class levels) Evil magic item or =<2nd 3rd–8th 9th–20th 21st or higher spell (caster level) 1 Except for undead and outsiders, which have their own entries on the table. 2 Some characters who are not clerics may radiate an aura of equivalent power. The class description will indicate whether this applies.
Does the paladin have the right and/or duty and/or option to attack and kill that stranger based solely on knowing his/her/its alignment?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.