Notions about absolute evil have been around for a while. So it's no surprise to find that various myths and religious scriptures provide a fine array of completely irredeemable villains, whether of divine or mortal origins. The page is currently split up alphabetically by mythos.
Greco-Roman myth is replete with examples of mortals who tried to outsmart the gods and paid for it. Yet none of them found as disgusting a method, or deserved their resultant punishment, as much as Tantalus did. A Greek king who was a favourite guest and host of Zeus', Tantalus proved that familiarity breeds contempt. Deciding that the gods were fools rather then omniscient beings, Tantalus set out to show that they could not even tell what they were being served for dinner, let alone comprehend the secrets of the cosmos. Murdering his own son, Pelops, Tantalus had him boiled, and served to the gods as the main course at dinner. By tricking the Olympians into eating their host's child, he sought to demonstrate that the gods were utter imbeciles. Instead, he proved that he was a relentlessly vile person and that even Olympian godshave standards. When his ploy was uncovered, Zeus both resurrected Pelops and trapped Tantalus in an Ironic Hell, cementing cannibalism, the slaying of one's own children, and the mistreatment of guestsnote see xenia, the Ancient Greek code on how to treat guests as major taboos in Greek culture.
Following in Tantalus' bad, bad example is Lycaon, tyrant of Arcadia, and the first werewolf. Appearing in several compendiums of myth from the time period, the most infamous version of Lycaon's story appears in Ovid's Metamorphoses. A cruel, bloodthirsty ruler, Lycaon was angered when his citizens celebrated Jove's (Zeus) visit to his city. Determined to prove that Jove was not really a god, and was therefore unworthy of their worship, Lycaon offered Jove a room for the night, plotting to murder him in his sleep. He also, like Tantalus before him, sought to test Jove's divinity, by serving the god human flesh at dinner. Butchering a hostagenote in some earlier versions of the story, it is his own son or infant grandson, Lycaon roasted his flesh, ate some of it himself, and placed it on the table before Jove, who reacted in outrage and disgust, cursing him with lycanthropy. Watching the newly lupine tyrant slaughter a herd of sheep, Jove noted that Lycaon's nature had not changed; it was simply that he was now as savage on the outside as he had always been on the inside.
Apep, more widely known in the West by his Greek name, Apophis, was the Egyptian God of Evil, and was associated with darkness, chaos, and destruction. Residing in the Underworld, Apophis took the form of a colossal snake, and every night, would attempt to devour the sun god Ra, as he made his way through the land of the dead, hoping to leave the world without light or life. During the day, while Ra was in the sky, Apophis would feed on those dead souls who were not properly protected against him, leading to their complete obliteration. His motives for doing so were that, as a primordial being associated with the dark, he hated the fact that there was a sun, or life on earth, and aimed to go back to the way things used to be. He was also the leader of armies of demons, and sought to subvert mortal confederates into aiding his nihilistic agenda. Reviled by all the other deities, including Set (who eventually took on the role of God of Evil), and universally loathed in Egypt, Apophis was the only god to have the distinction of never being prayed to. Instead, entire temples and ceremonies were set up to pray against him.
In the Book of Esther, Haman is a treacherous advisor offended by one Jew's refusal to bow to him, using this as justification for plotting the genocide of the Jewish people. Various Judaic traditions elaborate on this, noting Haman had a picture of an idol embroidered onto his robes so that he could force Jews bowing to him to violate the taboo against kneeling to idols. The very trees from which he hoped to build his gallows recoiled from his unclean presence, and unlike some Biblical antagonists, he's not an agent of divine retribution, just a self-important bastard who can't accept being disrespected. Even Haman's wife tells to his face that he has gone over the line, and that his obsession with killing the Jews would soon become self-destructive; he ignores her and goes ahead with his plans. His name has become a watchword for anti-Semitism and he is viewed in rabbinical tradition as an archetypal evil figure.
Angra Mainyu, alias Ahriman, is the Evil Twin of creator god Ahura Mazda, and the antithesis of everything good and just. Declaring that "it is not that I cannot make anything good, but that I will not," Angra Mainyu sought to prevent Ahura Mazda from creating life in the first place, by slaying the primal bull (his first creation). He seduced the daevas (demons) away from Ahura Mazda, transforming them into spiteful beings living only to spread fear and hatred. When Ahura Mazda created sixteen lands, Angra Mainyu responded with sixteen scourges, including old age, disease, war, vice, and death. He created the monster Aži Dahāka, destined to slay one-third of the earth's population, and turned him loose in the world. He made two separate attempts at destroying the Earth's water supply and leaving all creation to die of dehydration. He made Jeh, the primal whore, so that women would suffer from menstruation, birth pangs, and mistrust from men. He tried to coerce Zoroaster into coming over to his side, and then set the daevas upon him when he would not. Responsible for the very existence of evil, Angra Mainyu works to seduce men and women away from Ahura Mazda, so they might join his campaign for control of reality. Hating everything good and just, and angry that he is not the one who created everything, Angra Mainyu's ultimate goal is the annihilation of the universe.