The true motive of Polydectes in sending Perseus to kill Medusa is not explicitly stated in Greek mythology, but there are several possible explanations. Some of the possible motives are:

• Jealousy: Polydectes may have been jealous of Perseus divine parentage and his potential to inherit great power. Perseus was the son of Zeus, the king of the gods, and Danaë, a beautiful mortal princess, while Polydectes was a mortal king.

• Revenge: Polydectes may have wanted revenge on Medusa for turning him into a monster.

• Harm Perseus: Polydectes may have wanted to harm and kill Perseus by sending him on a dangerous mission. Polydectes believed that Perseus would not survive the mission, leaving him free to marry Danaë without any interference.

• Test Perseus: Polydectes may have wanted to test Perseus courage, strength, and intelligence by sending him on such a dangerous quest.

Whatever his true motive was, Polydectes decision to send Perseus to kill Medusa was treacherous. Medusa was a gorgon, a monstrous creature with snakes for hair and a gaze that could turn men to stone. Despite the danger, Perseus accepted the quest and managed to slay Medusa with the help of the goddess Athena. Perseus then returned to Seriphos and used Medusas head to turn Polydectes and his men to stone.