I'm about to geek out. You have been warned.
There are many versions of the story that depict her as a wicked woman who deserved to die. But I like to believe the story where she is the victim of a horrible injustice.
In Greek mythology Medusa was an ordinary girl blessed with the gift of beauty. She often spent her time in the temple of Athena as a chaste priestess. One horrible day Poseidon the god of the sea raped Medusa in the temple. Enraged by this act, Athena put a terrible curse on Medusa to punish her. She turned her flawless skin into rough and scaly skin. She transformed her perfect hair into a head full of venomous snakes. Who ever looked at this now hideous creature would turn to stone.
Medusa fled to a nearby cave where she would soon meet her death. Perseus (one of Greek mythologies well known heroes) found her sleeping in the cave and cut off her head. He used her head in battle to turn his enemies to stone before giving it to Athena as a gift. Which she then put on her shield.