Polydectes in Perseus and Medusa
Polydectes is the King of Seriphus, who decides he wants to get it on with Perseus' mother, Danae. It is Polydectes who demands that Perseus bring him back the head of Medusa, hoping that Perseus will die during the quest. In the end, Perseus does bring him the head of the Gorgon – but the hero uses it against Polydectes and turns him into stone. Sure, he's the bad guy in the story, but if it weren't for him, Perseus wouldn't be the legendary hero that he is.