I was watching the best friends, one of their recent "shitstorm of scariness" episodes, and the game they are playing is "medusa's labyrinth." Matt mentions briefly the myth of medusa and how she was turned into a monster for being vain and proud of her beauty.
No. That's not how it went down. Nothing against the best friends, more against the political correcting of this old story. Here's what actually happened, in a nutshell.
Medusa was a priestess of Athena. She was very beautiful, and everyone desired her but like all priestesses of Athena, she was under an oath of celibacy. One day, Poseidon looked down at Medusa in Athena's temple and decided he needed to get his dick wet, whether she wanted to or not and Medusa's oath to Athena be damned. So Poseidon went down to the temple of Athena and raped her, basically on Athena's altar. Then Athena cursed her to become a monster because "Oh Poseidon. Gods will be gods I guess, and gods are above reproach!"
TL:DR, Medusa was turned into a monster for the crime of being raped by Poseidon.