Ex-paladin

An ex-paladin is a paladin or paladin warrior who ceases to be good, who willfully commits an evil act, or who grossly violates the paladin code of conduct. A paladin must be of good alignment and loses all light abilities if he ever willingly commits an evil act. He loses all paladin spells and abilities (but not weapon, armor, or shield proficiencies).

Additionally, a paladin’s code requires that he respect legitimate authority, act with honor (not lying, not cheating, not using poison and so forth), help those in need (provided they do not use his help for evil or chaotic ends), and punish those who harm or threaten innocents. While he may adventure with characters of any good or neutral alignment, a paladin will never knowingly associate with evil persons, nor will he continue an association with someone who consistently offends his moral code. A paladin may accept only henchmen, followers or cohorts who are of good alignment.

If they break any of these, they may not train any farther as a paladin. He regains his abilities and advancement potential if he atones for his violations. An ex-paladin may only restore his paladinhood if he is truly repentant and desirous of setting right his misdeeds, this atonement requires undertaking a quest, and/or through the spell of atonement.

The Scarlet Crusade has a couple of ex-paladins dedicated to training new paladins who join the organization. Although paladins keep joining — and some are already a part of — the Scarlet Crusade, few can balance their commitment to the Light with the dark deeds done by the Scarlet Crusade, so few keep their holy status. Most abandon the teachings of the Light and find their abilities changing to suit their new mindsets. Many of these ex-paladins become scarlet knights.

Even many members of the Argent Dawn are known for having cast off paladin armor to find ways to turn the Holy Light toward destructive uses.

Even the most honorable paladins can fall prey to the temptations of the Lich King. Some who devote themselves to his cause become the horrors known as death knights — elite members of the Scourge who lead the Lich King's armies.

Famous/Named

 * Aretain Naris
 * Jorad Mace (formerly)
 * Farseer Nobundo
 * Kaylaan the Lost
 * Exarch Maladaar
 * Fallen Champion
 * Eldritch Deathlord
 * Thomas Thomson
 * Morgan Ladimore
 * Commander Springvale

Death knights

 * Arthas Menethil
 * Darkreaver
 * Highlord Mograine
 * Highlord Darion Mograine
 * Sir Zeliek
 * Aurius Rivendare

Note: An ex-paladin is not so much a true variant or class, but a penalized paladin (blocked from Light-given abilities).

Speculation
Ex-paladins may also include those who are ex-communicated.

It seems that corruption caused by external sources may also cause an individual to cease being a paladin, as was the case with Nobundo and Arthas Menethil. Although some sources imply Arthas was already making negative choices even before he took Frostmourne and lose his soul, such as the purge of Stratholme.

Ex-paladins may become scarlet crusaders, scarlet inquisitors, or death knights.