Why did Greece have Ares? There was no need for multiple gods of war, as Athena covered that space quite nicely and was definitely better-liked than Ares. He has no other major qualifications for relevance, other than having been born earlier than Athena was. The other gods, even his own father, openly dislike him, with Zeus even openly telling him in the Illiad that if he were the son of some other god, he would've been punted from Olympus long ago. The only deity shown to have positive relations with him is Aphrodite, and the exposure of their affair resulted in ridicule for both of them, so i doubt that lasted. He's the god of bloodlust, slaughter, needless violence, and indiscriminate killing on the battlefield. He's pretty much as close as you can get to an actually "evil" deity in Greek myth, portrayed with effectively zero redeeming qualities until he was syncretized with the FAR more dignified and diverse Roman equivalent, Mars, who was an agricultural deity and not NEARLY as closely linked to mindless slaughter.
But we don't even need to go that far from ancient Greece to find another real-world "god of evil." Just a short ways to the east, in Asia Minor, you have the Avestan/Zoroastrian tradition, which is specifically near-dualist (depending on time and place; sometimes Angra Mainyu is presented as almost but not quite Ahura Mazda's equal-and-opposite, other times Ahura Mazda is objectively superior in all ways and Angra Mainyu is foolishly wasting his time trying to win an impossible battle.) You also have the once quite widespread but now defunct Manichaeism, which is expressly dualist, with an eternal World of Light led by a good ultimate deity and an eternal World of Darkness led by an eternal evil one, with all of creation being just a byproduct of the constant wars of aggression perpetrated by the Dark against the Light.
So...sometimes, yes, you really can have actual deities that most people despise or fear, that are "on the prayer roll" to some extent and which are recognized for their power even if not for any positive qualities they might have. D&D definitely makes a lot more "evil god" type entities than is typical for most real-world religions. However, it also makes those gods very personable and often literally actually human beings who ascended to godhood, rather than demigods by birth who shed their mortality or gods who forgot their status and regained it, which is similarly unprecedented in mythology. So, given that the nature and process of godhood is already quite a bit different from how ancient peoples understood the divine, I don't really know that this particular divergence stands out as needing change, unless you're proposing to heavily rewrite how divinity works in a D&D context (as, for example, Rich Baker did with Eberron's deities.)
