The Yang principle. Literally meaning "sunlight", it represents fire, heat, aggressiveness and war.
In grimoires and western esoteric tradition, the Sun is associated with pride and arrogance. Leo at it's worst is a hubris-dominated little shit, or an outright tyrant. In Indian astrology, the Sun (represented by the god Surya) is a "malefic" planet.
Bila from Australian aboriginal mythology is a sun that also happens to be a cannibal, roasting her victims over a fire. That's where daylight comes from!
In Indo-European mythologies, the dawn goddess is generally a figure of ambivalence, as dawn shortens our days, placing us closer to death with each sunrise. In Hindu Mythology, the dawn goddess Ushas is outright stated to make us old, while in Classical Mythology Eos asked Zeus to make her lover immortal, but forgot the old age part, and as such he withered to a cricket.
In Brasillian folklore, the Boitatá is a serpent of light that incinerates or blinds those that it comes across. Legend says that, when the earth was plunged into darkness, it feasted upon the eyes of those who couldn't see in the dark, and that is why it became a luminous creature. On the plus side, though, it did end said night...
Sekhmet. Aside from Apophis, she is the most hostile of the Egyptian deities. Her most famous myth is basically her slaughtering half of mankind in name of Ra, who had to resort to drug her to stop the rampage. She's associated with the midday Sun and the desert, symbolizing the destructive and sterile power of light. Set to some extent as well, as he too is associated with the desert and the harsh Sun.
Archons are the gnostic equivalent of angels, and one, Adonaios, is associated with the Sun. Since Gnosticism has God Is Evil as a core philosophy, the archons are unsurprisingly considered malevolent or at least hostile.
As in the Percy Jackson and the Olympians example above, Hyperion, the titan of light, can be interpreted as this, since the titans were enemies of the gods and he participated in the Titanomachy. However, some see the Titans as better than the Olympians, seeing as they brought a golden age to man, though most likely both groups were neutral and Hyperion was prone to this.
According to Hesiod and Homer, Ares is this.
According to some theories, the word "titan" comes from the word "τιτανος", which is another word for "white", more specifically white earth. The myth where this connotation is most obvious is in the Orphic myth of Zagreus, where Zeus' intended heir, as a child, is lured out by the Titans who, dressed in white and with white paint on their faces, proceed to dismember and cannibalize him.
Zeus, the father of the gods, is a fantastic example of this. While many Sadly Mythtaken portrayals depict him in a similar light to YHWH as a benevolent father figure and just ruler of Olympus, the original Greek myths make it clear that he had sex with everything and used his powers to get out of getting caught with his pants down as best as he could, and that he frequently couldn't care less for what might be good and just, as long as he got what he wanted. He's not exactly evil per se, but the myths do definitely portray him as seriously flawed.
Apollo as well: Apollo, who is often referred to as the god of light, often pulls dirty tricks to get his way, and at least once arranged for some mortals to commit a heinous act, but then abandons them to their fate shortly after committing said heinous act (e. g., Elektra and her brother avenging their father by killing their mother and her lover).
Circe is a daughter of the Sun god Helios, yet she is consistently portrayed as an antagonist. In addition, it was claimed that witches had a power called "evil eye", which was derived from Helios (who was also the god of sight).
Medea, Circe's niece and grand-daughter of Helios through her father, was the original Yandere and actually used the power of the "evil eye": when an indestructible bronze giant attacked the Argo, Medea looked in his eyes and caused him so much agony he killed itself.
Neto, a pre-Christian Lusitanian (aka Portugal south of river Douro) god, was a god of the sun but also a war god, presumably as an acknowledgement of the sun's deadly side. The other light related god in the Lusitanian Mythology, Endovelicus, was supposedly good, though once Christianity settled in he became associated with Lucifer.
Would be Lucifernote actually "Heylel" in Hebrew, but it means the same thing, or "Light-bearer" in Latin, a name for Venus as the morning star. Also in the Biblenote "And no wonder, for Satan himself masquerades as an angel of light." ~ 2 Corinthians 11:14, Satan can still appear to men as an "angel of light," making this Older Than Feudalism. According to tradition, if Satan appeared before a person, they couldn't help but fall down and worship him because of his celestial beauty. Just as in the Bible, when an angel appears to someone as something obviously not just a man, the first thing he says is either "Be not afraid" or "Don't worship me!"—as angels in other parts of the Bible tended to kill cities when they were sent. The Apocrypha combines Lucifer and Satan into the same entity, as if he did not have enough names, maybe just to drive the point home.
Also, it is worth noting that Lucifer is a Fallen Angel. Specifically, the angel of light, thereby making this trope Older Than Feudalism. This extends to some beliefs that demons look exactly like the winged angels that they used to be; having only changed inside when they fell, not outwardly. And don't really look like the hideous horned red-skinned beings as depicted. If an army of demons and angels fought each, there would be no telling the two sides apart.
Some refer to Sorath (the geomantic spirit of the Sun) as the Anti-Christ and true Bigger Bad behind Satan because his sacred number is 666. As it happens, there are no references of Sorath in The Bible. The number 666 in the Book of Revelation represents a being that is subordinate to Satan.
Even in the context of The Bible, the Levantine deity Moloch is correctly identified as a solar deity. According the bible, he demanded child sacrifices. However, little is known about the deity to know who he is exactly, and Archeologists are doubting what legends are saying about him.
It's in more than just the folklore, it's canon (in the both senses of the word!)
The Abrahamic God is viewed by some religions as their God of Evil, as is the case in Gnosticism.
The Bible also has a villain named Laban, which means "white." One Jewish interpretation is that this name is better understood as white as in "blank"—Laban was a Manipulative Bastard with no real moral code, but because of this, he could easily put on whichever one was currently useful (businessman, family man, etc.)
"Lady Midday" from the Slavic Mythology. Unless you consider sun-stroke a good thing.
Lakota mythology features light and darkness as more reason and chaos. But following this theme, Yata, the north wind, is represented by white. He also causes harsh winters. Also, the wakinyan, or lightning gods, are either this or Good Is Not Nice.
While the Celtic conflict between Balor and Lugh has been interpreted traditionally as the generic Dark Is Evil versus Light Is Good, things seem to have really been inverted: Balor seemingly was associated with the harsh Summer Sun, represented by his "evil eye" (see above), while Lugh was a thunder god that brought on the clouds and rain.
His Welsh's counterpart's name, Gronw Pebr, means "Gronw the radiant"...
Adlaw/Apolaki, the Phillipine god of the Sun, fought against his sister for supremacy of the world, refusing to share it with her, and even took out her eyes so that he could be the single ruler. He's also associated with war.