False Paladin (3.5 Paladin Variant)
I've always liked the idea of making paladins villains, but it's sort of hard to do so (excluding evil parties, in which case the paladins still aren't really "villains.") for the obvious reason that the paladins would either fall, or wouldn't be evil enough to truly be BBEGs. This variant is pretty much giving crunch to an idea I've had for awhile.
Paladins are, ideally, shining beacons of good and hope to the world. But they must also face evils few can even imagine on a daily basis. While most paladins retain their faith and continue to act as they know they should, the pressure drives some paladins to believe more...extreme measures are required to stop evil. Some fall, but sometimes, when a paladin has gone truly off the deep end, their unshakeable belief that no matter how evil what they're doing appears to any other entity in the multiverse, they are doing the most proper and good thing imaginable. Their sheer faith in their own beliefs continues to give them powers similar to a paladin's, but warped by their madness.
Aura of False Good: Beginning at 1st level, a False Paladin will appear on Detect Good as having an aura of good equal to their level. They will also appear as Lawful if a Detect Law spell is cast on them. This is purely illusory, and a successful caster level check (DC = 10+False Paladin Class Level+Cha Modifier) will reveal their true alignment
False Prestige: For the purposes of taking prestige classes, a false paladin is considered Lawful Good, and his False Paladin class features fulfill any requirements for the equivalent Paladin class features.
Detect Enemy: At first level, the False Paladin gains the ability to detect those he views as evil. This functions as Detect Evil but is warped by the False Paladin's madness, revealing any one he personally would view as "evil" as being of evil alignment. To use an OOTS example, Shojo would've detected as evil to Miko if she'd been a False Paladin.
Smite Enemy: The False Paladin can use the Smite Evil class feature, although it functions based on the same principal as Detect Enemy, rather than smiting only those who are actually of evil alignment.
Spells: The False Paladin casts the same spells as a paladin, except any spell specifically affecting those of evil alignment (Detect Evil, etc.) instead affect anyone the paladin views as "evil."
I realize a lot of this is sort of undefined, and it's meant to be. All False Paladins are a little different, and some are significantly worse than others. For instance, let's say we have a CN False Paladin who has come to value freedom of any kind as "good." Therefore, random LN town guards will show up as "Evil" using the "detect enemy" class feature.
On the other hand, let's say we have a LE False Paladin who views alcohol as a gateway to sin and chaos. A Bartender would show up as "evil" to her, but town guards would not.