Top 7 Worst Betrayals in Greek Mythology

1. Jason and Medea

Jason and Medea both violated each other's expectations. Jason had lived with Medea as her husband, even producing children, but then put her aside, saying they were never married, and that he was going to marry the local king's daughter.

In retaliation, Medea slew their children, and then flew away in one of the classic instances of a deus ex machina in Euripides' Medea.

There was little doubt in ancient times that Medea's betrayal was greater than Jason's.

2. Atreus and Thyestes

Which brother was worse? The one who engaged in the family sport of cooking children or the one who first committed adultery with his brother's wife and then raised a son for the purpose of killing his uncle? Atreus and Thyestes were sons of Pelops who himself had once been served up as a feast to the gods. He lost a shoulder in the event because Demeter ate it, but he was restored by the gods. Such was not the fate of the children of Thyestes whom Atreus cooked. Agamemnon was a son of Atreus.

3. Agamemnon and Clytemnestra

Like Jason and Medea, Agamemnon and Clytemnestra violated each others' expectations. In the Oresteia trilogy the jury couldn't decide whose crimes were more heinous, so Athena cast the deciding vote. She determined that Clytemnestra's murderer was justified, even though Orestes was Clytemnestra's son. Agamemnon's betrayals were the sacrifice of their daughter Iphigenia to the gods and bringing back a prophetic concubine from Troy.
Clytemnestra (or her live-in lover) murdered Agamemnon.

4. Ariadne and King Minos

When the wife of King Minos of Crete, Pasiphae, gave birth to a half man, half-bull, Minos put the creature in a labyrinth built by Daedalus. Minos fed it the youth of Athens who were paid to Minos as annual tribute. One such sacrificial youth was Theseus who caught the eye of Minos' daughter Ariadne. She gave the hero a string and a sword. With these he was able to kill the Minotaur, and get out of the labyrinth. Theseus later abandoned Ariadne.

5. Aeneas and Dido (Technically, not Greek, but Roman)

Since Aeneas felt guilty about leaving Dido and tried to do so secretly, this case of jilting a lover counts as a betrayal. When Aeneas stopped at Carthage on his wanderings, Dido took him and his followers in. She offered them hospitality and in particular, offered herself to Aeneas. She considered theirs a commitment like a betrothal if not a marriage, and was inconsolable when she learned he was leaving. She cursed the Romans and killed herself.

6. Paris, Helen, and Menelaus

This was a betrayal of hospitality. When Paris visited Menelaus, he became enamored of the woman Aphrodite had promised him, Menelaus' wife, Helen. Whether Helen was in love with him, as well, is unknown. Paris left Menelaus' palace with Helen in tow. To regain Menelaus' stolen wife, his brother Agamemnon led the Greek troops to war against Troy.

7. Odysseus and Polyphemus

Crafty Odysseus used trickery to get away from Polyphemus. He gave Polyphemus a goatskin of wine, and then poked out his eye when the cyclops fell asleep. When Polyphemus' brothers heard him roaring with pain, they asked who was hurting him. He answered, "nobody," since that was the name Odysseus had given him. The cyclops brothers went away, mildly puzzled, and so Odysseus and his surviving followers, clinging to the under-bellies of Polyphemus' sheep, were able to escape.