While translating the spell-less Paladin variant from Unearthed Arcana to Pathfinder, I decided to make some Paladin variants based off other alignments. I've never been terribly fond of the single alignment class, but fortunately the Pathfinder Paladin has abilities that are quite easy to swap for other alignments. In case any of you are interested, here goes:
Alignment Variants
Standard Paladins may no longer apply the axiomatic ability to weapons under Divine Bond. Paladins may now apply the vicious ability to weapons under Divine Bond.
The standard Paladin may be any-Good, and aside from the above note functions in an identical manner to the standard Pathfinder Paladin. As Paladins are not tied to an ethical alignment, Paladins need not adhere to a code of conduct regarding authority and lying, but a player of a LG Paladin may still choose to adhere to such restrictions.
Paladins may also choose to be any-Evil, in which case all references to Good or holy in any spell or ability descriptors should be replaced with Evil or unholy, references to evil in abilities should be replaced with good, and other appropriate abilities should be replaced. Thus, detect evil becomes detect good, Divine Bond may now add unholy instead of holy to weapons and the mount's celestial template should be replaced with fiendish. Other conversions should be relatively straightforward and obvious.
Paladins may also be any-Lawful or any-Chaotic. For Lawful Paladins, all normal descriptors of good should be replaced with law/lawful, holy with axiomatic, evil with chaotic, etc. and the opposite for Chaotic Paladins. The slight complication occurs for mounts as no axiomatic or anarchic template exists in the rules, and the one listed in other D&D supplements are more powerful.
Fortunately, fiendish and celestial are remarkably similar to each other except for flavour, so we can simply steal from them and use them as lesser anarchic and lesser axiomatic templates. Both templates grant darkvision 60 ft., smite opposing ethical alignment, DR 5/opposing ethical alignment and SR equal to new CR +5. Lesser anarchic creatures gain no energy resistances but gain fast healing 1. Lesser axiomatic creatures gain energy resistances to cold, electricity and sonic.
Some suggestions for appropriate action restrictions (other than leaving the alignment) are:
Good: Must help those in need, must not commit evil acts, must punish those who do evil, and should not associate with evil characters.
Evil: Must not do honestly selfless or good acts. Such acts are those which are not only apparently good acts, but those intended to have good outcomes and not mere stepping stones towards an evil goal. An Evil Paladin must be careful not to do genuinely good acts, or he may risk losing his status.
Law: Must respect legitimate authority, must not perform acts which promote lawlessness or seek to bring down a legitimate authority, must not engage in falsehoods, as well as deep adherence to any other specific codes or religious requirements (such as never raising an arm against your lord even in self-defence). Such specific codes may vary per Paladin depending on her training.
Chaos: Must not support or perform actions which might support rigid hierarchies, rules and authorities, must encourage freedom, autonomy and independence.
Note 1: Regarding the any-Good allowance, no mention of Law is mentioned in *any* Paladin ability, and only really appears in a brief blurb in the code of conduct. Given that, it seems that the only thing sticking the Paladin to LG is tradition. You may wish to call non-Good Paladins something else, such as Blackguards for Evil Paladins, Crusaders of Freedom for Chaotic Paladins and Justicars for Lawful Paladins.
Note 2: Some might consider Auras to be specifically Good or Lawful in nature, but as Paladins under this system can be considered mortal exemplars of their alignment, it stands to reason that they should be promoting their alignment in a similar scale. An Evil Paladin can still be Evil whilst buffing their allies or minions.
Note 3: I don't typically identify good = positive energy and evil = negative energy. I find doing so makes no sense from the perspective of cosmology, where the energy planes were inner planes, contrasts with the way energy has been typically represented in settings such as planescape, and also artificially limits possibilities for no good reason. The good-aligned mummy who guards his tombs and the evil Tanar'ri/demon who uses positive energy creates evil humans within the wombs of a nation are both interesting concepts to me. Plus, I don't see why you couldn't have a militant Good Paladin who smites his (humanoid) foes with negative energy, or the undead-hating or leader of his fellow (evil) men Evil Paladin who keeps his minions alive in battle so that they may better kill his targets.
Note 4: I'm somewhat dubious in allowing a Paladin to embody two alignments at once. I'll give a brief outline on how you may do it at the end, but I don't really support doing it.
Negative Energy Variants
Paladins may choose to channel Negative Energy rather than Positive Energy. Negative Energy Paladins channel Negative Energy at level 4, instead of Positive Energy. Their Lay On Hands heals 1d6 points of damage only on Undead, and it can be used as a melee touch attack against living creatures (or Deathless) dealing 1d6 points of damage, in the same way as a standard Paladin may do so against Undead.
The Mercies gained at level 3 onwards are Cruelties for a Negative channelling Paladin, and inflict the intended effect against a living target instead of curing it.
The Diseased effect replicates the effect of the contagion spell and and the Poisoned effect replicates the effect of the poison spell. All other cruelties last for one hour unless otherwise cured. The target receives a Fort save to negate against a DC equal to (Paladin’s level + Cha modifier + half the level required to select the Cruelty used) unless otherwise noted. Negative Energy Paladins may take the Extra Cruelty feat, which functions like the Extra Mercy feat, but for Cruelties.
Note: The Negative Energy variant is easily combined with the above variants, such that all manner of combinations are possible for Paladins. A Paladin (Evil, Negative) represents your standard D&D Blackguard, a Paladin (Good, Positive) represents your standard D&D Paladin, and lovely combinations like Paladin (Law, Negative) could represent someone in charge of smiting lawbreakers, such as Vhailor.
Spell lists for both variants might need some revision, but as I originally made this for a spell-less version I haven't put much work into it. On the face of it, though, a standard alignment replacement should work, with smite evil becoming hammer of justice for a Paladin (Law), and cure spells becoming inflict for a Paladin (Negative), for instance. The only change for standard Paladins is that under this, normal Paladins would only gain protection from evil, and not protection from evil/chaos, along with similar modifications to account for no longer having access to anti-Chaos/pro-Law abilities. Other changes are always plausible as well, such as darkness instead of daylight for a Paladin (Negative), and so forth.
Double-alignment Variant
Finally, the double-alignment variant. I don't really recommend it as it still seems a bit powerful, and also violates the standard DR progression. Not violating the DR progression makes the Paladin very powerful though, as they would either have two DRs that must be defeated , or worse, one DR which is much harder to defeat at a higher value than below.
Paladins may also choose to embody two alignments rather than one (e.g. Law and Good rather than just Law as a Lawful Good Paladin), in which case they can use Detection and Smite powers against the two opposing alignments, and gain DR 3/ethical alignment and moral alignment at level 17, with DR 6/ethical alignment and moral alignment at level 20 (so a level 20 Paladin (Law, Good) would have DR 6/chaotic evil). A Paladin embodying two alignments may use both appropriate Smite abilities, but the total is shared by the two Smite abilities, so a level 10 Paladin (Law, Good) could use Smite Evil and Smite Chaos a total of 4 times a day split between them. Additionally, they may only use detect [Alignment] once per turn for each of their alignments, so a Paladin (Law, Good) may use detect chaos and detect evil each once per turn. A double-alignment paladin who chooses a mount may choose either of her alignment's templates to apply, but may only apply one.