A crown is a metonym (a kind of metaphor) for rulership. Medea
chooses to exact her revenge on Creon
and the Princess
with a poisoned crown, a crown that represents Creon and Jason
's having polluted the royal line with unjust rulership. Creon has earned his death and the loss of his authority by punishing the innocent Medea, who he himself has wronged by encouraging Medea's husband to abandon her and marry Creon's own daughter. Jason
has earned his grief by violating his marriage oaths and abandoning his wife and children for the promise of future ruling power.