What was the evil power of Medusa?

She would turn everything that she saw into stone

So people would not look into her face

Her evil powers were to turn men into stone was the result of her punishment from Athina. Athina punished her because Medusa was a priestess at the Parthenon. Neptune and Athina hated one another( they battled for the honor of having Athens named after one of them). Neptune seduced Medusa and ravaged her in the Parthenon. Athina got back at Neptune by punishing Medusa. Turning her into a creature that no man could love and she could love no man. Her evil powers were evil due to Athina.