Why did medusa turn people to stone?

Answer 1

Medusa turns people to stone because the goddess Athena caught Medusa and Poseidon in Athena's temple making love, so she cursed Medusa to become ugly, and her curse also included turning anyone who looks into her eyes to stone.
Because she was so ugly, for lack of a better word.

^^ -.- pretty much

Medusa was originally a ravishingly beautiful maiden, "the jealous aspiration of many suitors," priestess in Athena's temple, but when she and the "Lord of the Sea" Poseidon lay together in Athena's temple, Athena was enraged so she transformed Medusa's beautiful hair to serpents and made her face look so terrible if anyone were to look into her eyes directly, it would turn onlookers to stone. In Ovid's telling, Perseus describes Medusa's punishment by Athena as just and well-deserved.

that's y :)