Dalaran Human: Kel'Thuzad - dude was very evil. he is most evil human ever.
Lordaeron Human: Arthas was more like dumb and stubborn guy who got himself cursed. while KT choose his road willingly and enjoyed it
Orc: Gul'Dan. both versions. evil incarnate
Quel'Dorei: Dar'Khan Drathir - also power hungry guys who sold his own people and country to real evil for more power
Gnome: Mekgineer Thermaplugg - for personal gains and animosity vs Gelbin, he made Gnomes irradiate Gnomeregan, thus killing it's people and irradiating remaining ones (including himself)
Worgen: do not seem to have anyone evil. they are mostly result of tragic circumstances
Kul Tiran Humans: Lady Ashvane - manipulator, power hungry and ruthless bitch. betrays her own nation and any other ally to anyone for her personal gains. also she uses child labor
Gilnean Humans: nothing much here. Lord GOdfrey, who in his blind hatred for worgen, doomed his own nation by betraying his own king + allying with forsaken and betraying them as well
Alterac Humans : Lord Perenolde, who sold Human Allaince to Orcish horde and later allied himself to returned Horde again
Stromgarde Humans: Prince Galen, who killed his own father for crown, and resulted kingdom to be left in ruins. later raised by forsaken and betrayed them as well
Stormwind Human: they do not have such absolute evil guy
Wildhammer Dwarves - none here
Dark Iron Dwarves - Queen Modgud, who, using dagger of Black Empire, helped her husband, king Thaurissan orchestrate new war against other 2 clans
Mechagnomes - King Mechagon
Lightforged Draenei - none
Void Elves - none
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Jungle Trolls - Jin'Do the Hexxer
Forest Trolls - none are absolute evil
Sand Trolls - General Jakra'zet
Dark Trolls - none
Ice Trolls - High Priestes of Loas, they killed their own loas for more power vs Lich King
Zandalari Trolls - Zul. idk what kind of Prophet he was, but he betrayed his own nation, king and wanted all to be damned under Old God powers
Ogre - Cho'Gall
Forsaken - any