Great question, and props for recognizing the influence of Classical mythology in particular in the art and literature of Western Civilization!
This answer may evolve, as I get more info on what you've looked at, but I'll initially address the subject of cannibalism in Greek Myth, since there are so many referents.
When Zeus visited his home, all the people worshipped Zeus, but Lycaon,
"mocked their pious vows and scoffing said; ‘A fair experiment
will prove the truth if this be god or man.’
SOURCE: Metamorphoses, Ovid, Book I
Lycaon's experiment is to kill and cook a Molossian hostage, and serve him to Zeus to test his power. Needless to say, the experiment fails, and Zeus destroys his house with lightening bolts. Lycaon is transformed into the creature that most fit his ravenous bloodlust, a wolf. *(In the Ovid, Zeus is so freaked out by his run-in with the original "Hannibal Lecter", he wants to destroy all mankind, no longer feeling so secure about his dominance.)
Lycaon was undoubted wicked and impious!
- Tantalus, honored by the gods, intimate friend of Zeus, regular guest at Olympus.
Tantalus first sinned by stealing some nectar from one of their feasts, and before the crime could be discovered, added insult to injury. Having invited the gods to dine at his palace, Tantalus discovers his larder is empty. Not wishing to offend, slaughters and cooks his son Pelops. All of the gods know exactly what was on the menu except for Demeter, who, distracted by the absence of her daughter, eats a shoulder, resulting in one of the earliest prostheses when Pelops is subsequently resurrected. It's unclear whether Tantalus was testing the gods, or just trying to be a good host, but the gods punish him nonetheless.
Even in Hades, however, Tantalus has distinction of being the victim one of the most famous eternal punishments of all time, and his name becomes the word tantalize.
- Atreus, father of Agamemnon
Cannibalism is something of a tradition in the lineage of Tantalus, and extends to his grandsons. Atreus and Thyestes are two feuding sons of Pelops, and are wickedness defined. Thyestes has an affair Atreus' wife; Atreus, to get back at Thyestes for making him a cuckold, kills Thyestes' two sons, cooks and serves them at a feast to Thyestes, taunting him with their heads and hands after Thyestes has gorged himself. Thyestes then rapes his own daughter because of a prophecy that the son of the union would kill Atreus, which turns out to be true. No punishments were meted, presumably because because the outrages were against mortals, not the gods, except, possibly, in some curse on the house of Atreus.
Agamemnon has a lot of family baggage!
- Philomel and Procne, two princesses of Athens.
This is my favorite variant of the "Thyestian Banquet" because in this one, the actors are justified. The Ovid version is the most moving: Procne et Philomela (English translation).
Tereus, the husband of Procne, is wicked indeed. Not satisfied with his young, beautiful wife, he abducts her younger sister and locks her in a tower at the edge of his domains. There he has his way with poor Philomel, and cuts out her tongue so she can tell no one of his outrage outrage. But women have special domains of power in the myths, and one of these is weaving (see the Fates, Athena/Arachne, Penelope.) Philomel weaves a tapestry depicting her tale of woe, one which only her sister would recognize, and by signs and signals, has a servant to convey it as a gift to her sister, the queen.
Long story short, Procne gets the message, rescues her sister. For revenge, the two slaughter young Itys, Procne & Tereus' son, and cook him up and feed him to Tereus, except for the head and hands which, like Atreus, they use to reveal the contents of the feast once Tereus has stuffed himself so full he can barely move.
All are punished by being turned into birds, and it's easy in a modern context to see the sisters as wicked, but they're actually the heroines. Tereus certainly had it coming!
Titus Andronicus (Shakespeare)
The Cook, The Thief, His Wife, Her Lover (Greenaway)
These are just a few of my favorite tales of wickedness, and I chose them partly for the shared element of cannibalism, and partly because the actors all all protagonists. This makes these stories distinct from, say, the wicked men that Theseus kills on his first trip to Athens, such as Periphetes, Sinis, Sciron, Cercyon, and the inimitable Procrustes.