First, a bit of background. Conquering Agamemnon slew his daughter, Iphigenia, in return for a fair wind to Troy. This rather miffed his wife, Clytemnestra, who bashed his head in with an axe when he came swaggering home. Her retribution laid a religious duty on their son, Orestes, to avenge his dad by slaying his mum, which, in its turn, put a bit of a crimp in his social calendar.