Medusa was punished for her vanity

The argument

Medusa, in some iterations of the myth, is portrayed as a seductress. She knew how beautiful she was and used her beauty to her advantage. Medusa would seduce Poseidon and have a sexual encounter with him in the temple of Athena, a virgin goddess. When Athena found out about this encounter, she turned Medusa into a gorgon to punish her for her pride and the sins she committed against the goddess. In this version of the story, Medusa's actions are referred to as sin. One can argue that sin is a conscious transgression against a deity. Once an individual has sinned, this then prompts the offended deity to take action against said person, usually in the form of judgment. Since sin is considered evil, Medusa would thus fall under the category of evil. To further speak to this idea, another version of the Medusa myth elaborates on Medusa's sin in depth. This iteration sees Medusa admiring herself in the mirror, which is then followed by a remark stating that she was more beautiful than Athena. In the previous story, one may have made the case that committing sexual acts was not a justifiable reason to punish Medusa. This telling of the myth, however, makes explicit what Medusa's transgression against Athena was. In Greek mythology, comparing oneself to the gods often begets dire consequences. When Medusa compared herself to Athena and proclaimed herself prettier, she attempted trespassing into the realm of the gods, effectively making herself a target of Athena's judgment.

Premises

Counter-arguments

On the other hand, Medusa may have been punished by Athena for another reason. It may not have been the result of Medusa's vanity, but because of Athena's jealousy of Medusa's beauty. One of the common threads in most iterations of the Medusa myth is that Athena was jealous of Medusa. Finding Medusa in a relationship with Poseidon arguably gave Athena an excuse to punish her, enabling the goddess to act based on her predispositions. If Athena did not feel jealous of Medusa, it is possible that Athena would not have been so enraged at Medusa. Not having that impulse would likely have resulted in a different punishment, or even resulted in Athena directing her fury at Poseidon instead.

Rejecting the premises

[Rejecting P1] The versions cited do not say the same thing, and the argument needs them to. In Ovid's account Medusa is assaulted by Neptune in the temple rather than seducing him — the reading the sibling position rests on — while the seduction and mirror versions are later retellings. The argument selects among them without giving a reason for the choice. [Rejecting P2] Calling these acts "sin" imports a category from a different religious framework. Greek myth deals in hubris, pollution and offence against a god's honour rather than in sin, and the inference from "sin is evil" to "Medusa is evil" depends entirely on the borrowed vocabulary. Even granting hubris, the punishment is what is in question: a boast about appearance answered with permanent transformation and eventual beheading is precisely what makes the victim reading available, and the counter-argument's point about Athena's own motive goes unaddressed.