* Mordred in [[KingArthur Arthurian legend]] went from playing a small but important role as the killer of Arthur (and something of a WorthyOpponent) to becoming Arthur's [[BastardBastard evil illegitimate son]] who was connected to Morgan le Fay. In his earliest appearances, it isn't clear whether Mordred and Arthur were enemies at all - it is only said that Mordred fought in the battle in which Arthur was killed, without making it clear whether they fought on the same side or opposing ones.
** Morgan le Fay herself was, in her earliest incarnations, a healer who ''helped'' Arthur by preserving his immortality, not the evil witch seen in later versions of the story.
** A story that Arthur killed his own son in battle is actually one of the earliest recorded; presumably Mordred's villainy was developed to make this okay.
* Odysseus, while [[WorthyOpponent respected by many of his enemies]] for his [[GuileHero cunning and tactical skills]] in GreekMythology, was viewed as a liar and a cheat by the Romans (such as Creator/{{Virgil}}), who treated him as a pure villain and placed far less emphasis on his good characteristics. In ''Literature/TheDivineComedy'', Dante placed Ulysses in the hell of evil counselors.
** The Greeks from the Trojan War got this in general from the Romans and their later followers. The Romans were supposedly descended from the Trojans (specifically Aeneas), so naturally they weren't happy by the way their ancestors had been treated.
* Set in EgyptianMythology, although he feuded with Horus after killing Osiris, was originally the protector of Re from the evil serpent [[EldritchAbomination Apep]] and worshiped in his own right. After Egypt was split between the Upper and Lower Kingdoms, he became [[GodOfEvil an evil god]] in Lower Egypt and his positive aspects were handed over to other deities. His worship as the god of foreigners almost entirely stopped after the Hyksos invaded Egypt.
** He fell into decline as the trio of Osiris, Isis, and Horus rose in prominence. Different cult centers always had different opinions on everyone, of course. He was also associated with the desert, which seems to have gotten less awe and more resentment over time.
* Loki of NorseMythology, while always a trickster, steadily becomes more and more of a bad guy throughout the different stories. Some of this is assumed to be because of Christian influence.
** Mostly occurred due to later writers missing that there are two distinctly different entities in Norse mythology named Loki and then further conflating the stories and linking them incorrectly, creating even more confusion. On top of that, some of the myths written later appear out of nowhere and contradict the earlier versions. For example, in the original form of the myth about Baldur's death Loki is never even mentioned, let alone involved.
** There is also no reason to believe there was ever a standard canon, let alone one that synced up understandings across different population centers. A major theme of Loki in the recorded stories seems to be that '[[AnAesop it is dangerous to let someone into your kinship group who might cause trouble, even if he's useful and you like him a lot.]]' This is unlikely to have been part of his original conception.
* The gods of several religions show up in those of rival civilizations as demons.
** Though in this case it's less AdaptationalVillainy and more "[[HijackedByJesus giving some demons]] [[InNameOnly the names of rival culture X's deities]] [[TakeThat to denounce them.]]"
** Speaking of demons, {{Satan}}, probably the most famous demon of all, [[SatanIsGood worked with God]] in the original Jewish texts. While he did tempt people, [[SecretTestOfCharacter that was to test them.]] The New Testament decided to turn Satan into a force actively opposing God, [[{{Retcon}} retconning him]] to be [[BiggerBad the serpent who introduced sin in the first place.]]
*** Zoroastrian dualism was influencing popular religious understanding among its neighbors a lot around the beginning of the Current Era, and the traditional Hebrew approach of characterizing anything non-Hebrew as evil simply on the strength of it not being Hebrew, and therefore probably wanting to kill or take advantage of the Hebrews, was working less and less well in the civilized and cosmopolitan modernity of the first century. 'Badness' began to coalesce more into a ''figure'' rather than a force.
*** The early Christian sect/cult particularly needed a new definition of evil, since their recruitment among the Romans and other gentiles was huge, and xenophobia simply did not work as a basis anymore.
*** However, fusing every non-human and potentially non-human baddy mentioned in the texts into [[BigBad one being]] was a process that only ''began'' in the first centuries AD, and persisted all the way into the middle ages as Christian theology continued to develop.
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