Betrayer

Betrayer, while usually a more general term, is often applied to certain characters within Warcraft lore whose crimes are so heinous that they have been immortalized by their societies as traitors.

Alexstrasza
Called betrayer by Emberstrife for buying the other Aspects enough time to escape Nefarian at Grim Batol.

Anub'arak
Anub'arak was a former king of Azjol-Nerub who was raised as one of the Lich King's crypt lords. When leading Arthas into Azjol-Nerub in the race to Icecrown, they encountered living nerubians, who called Anub'arak "the traitor king" - which made Arthas think, at first, that they were referring to him.

It can be argued however that Anub'arak is just another helpless slave of the Lich King, as when he dies in Azjol-Nerub he utters, "I never thought... I would be free of him."

Deathwing
Once known as Neltherion the Earth-Warder, Neltherion fell into corruption by the Old Gods sometime during the War of the Ancients. He created the Demon Soul and used it not only on the Burning Legion, but the Night Elves and most of the Blue Dragonflight. While holding on to the Demon Soul too long his body was falling apart and his insides became fire. So the same Goblins that forged the Demon Soul forged armor for Deathwing that would keep his body from falling apart. It is also said that Deathwing is the main cause for war between the races because he ruined fertile land and thus encouraged races to fight over food.

Dar'Khan Drathir
Once an esteemed magister, Dar'Khan Drathir has become one of Quel'Thalas' greatest traitors. His limitless ambition, combined with his egotistical mindset, made him an easy pawn for the forces of the Lich King. Empowered by the treacherous Prince Arthas, Dar'Khan opened the way for the Scourge to enter Quel'Thalas and ravage the land in their march to the Sunwell.

Gul'dan
The evil orc warlock has been called "betrayer" and far worse by the Horde in the decades since his demise in the Tomb of Sargeras. The specific betrayal that most orcs remember dates back to around the end of the Second War. As Orgrim Doomhammer, Warchief of the Horde, prepared for his final assault on the Capital City of Lordaeron, Gul'dan took both his Stormreaver Clan and the Twilight's Hammer - led by his ogre-mage lieutenant, Cho'gall - and set out across the Great Sea to raise the Broken Isles from the depths. Enraged, Doomhammer's battle-hungry clans followed Gul'dan across the Great Sea and laid waste to the Stormreavers and Twilight's Hammer. Gul'dan himself was already dead when Doomhammer arrived, but the split in the Horde forces ultimately cost them the war. It could be argued, however, that Gul'dan's true betrayal was in his pact with the Burning Legion, in which he willingly condemned his entire race to slavery and servitude to gain personal power.

Arthas Menethil
Arthas is called a betrayer because of his horrible acts against the world. He brought Lordaeron and Quel'Thalas to ruin and slaughtered many innocents, including his own father, King Terenas. It should be noted though, that one could argue that he did not betray his people, because when he took up Frostmourne he wanted to save his people and rid the world of the Scourge. One could also argue that Muradin did tell him that his soul would be scarred. Nonetheless, it is generally accepted that Arthas betrayed his people by taking up Frostmourne and slaughtering many elves, humans and to a lesser extent dwarves.

Mennu the Betrayer
After betraying his kind to the naga under Lady Vashj's rule, Mennu the Betrayer and many fellow broken subordinates now guard the Slave Pens in Coilfang Reservoir. It is likely that many of the Broken found in cages throughout Coilfang Reservoir are there because of Mennu's treachery. Mennu is an example of a broken who has found a way to turn Illidan's reign of terror to their advantage, by betraying his fellow draenei he gained favor with his naga overlords. His betrayal has tainted his shaman training, and he now wields corrupted totems, which make him a formidable opponent.

Sargeras
The Dark Titan is often referred to as the Betrayer of Life or the Betrayer of the Titans. Originally a noble warrior who led the Titans' armies against the evils that existed within the Twisting Nether, Sargeras became corrupted and set himself to destroying the Titans' orderly universe. Thus was the Burning Legion born, and thus began the eternal Burning Crusade.

Maiev Shadowsong
The Warden who had been responsible for Illidan's imprisonment, Maiev had tricked Illidan's brother, Malfurion, into thinking that his lover, Tyrande Whisperwind, had been torn apart by the undead, in order to goad the Archdruid into action. In fact, Tyrande had fallen from a collapsing bridge and been swept up the river. When informed of Maiev's deception, Malfurion entangled her and asked who the true betrayer was.

Illidan Stormrage
"Betrayer" is a term often used to refer to Illidan. Specifically, it refers to how he had apparently joined Azshara against the Night Elves and his creation of a new Well of Eternity by taking vials of water from the old one and pouring them into an untainted lake. Illidan's other questionable actions since then have paled by comparison.

Kael'thas Sunstrider
Kael'thas Sunstrider was the proud prince of Quel'thalas. He promised his people the Blood Elves a new source of magic. After his failure at Tempest Keep he left Outland and joined an alliance with Kil'jaeden, overlord of the Burning Legion. He gave his people fel magic and it consumed them, transforming them into Felblood. Kael'thas then attempted to summon Kil'jaeden through the Sunwell, similarly to the way the Well of Eterity was used to create a portal to summon Sargeras into Azeroth.

Varimathras
Varimathras, Archlord of Undercity and Majordomo of Sylvanas Windrunner. During the Civil War in the Plaguelands Varimathras confronted Sylvanas thanking her for her help against Arthas and that she may either join the Burning Legion or leave the Plaguelands. Sylvanas declined both and forced Varimathras to join her in taking the City of Lordaeron for herself. Varimathras first betrayed all Nathrezim by breaking the number one rule, killing another Nathrezim. He killed Balnazzar and the Forsaken formed on the same day. Varimathras then joined with Grand Apothecary Putress in taking the Undercity and betraying the Horde at the Wrath gate. Traître