Medusa was a victim of sexual violence and the story you know made her a villain.
Medusa was a beautiful young woman and Poseidon desired her for himself. The god of the seas attacked her and raped her inside a temple dedicated to Athena.
The goddess, Athena, took this attack as an offense and punished the woman by giving her snakes instead of hair and with the curse of turning anyone who looked into stone.
When Perseus beheaded her, the giant Chrysaor and winged horse Pegasus sprouted from her neck. Both are considered the children of Poseidon, which means that they were the result of rape and that Medusa was pregnant when she was murdered.
Medusa is still remembered as a monster when her only "crime" was being attractive.
The victim was also the only one who received punishment for Poseidon's acts.
Medusa turned out not to be the real monster in this story.