Is Orestes justified in killing his mother? Why or why not

The Chorus of Furies interrogate Orestes about his crime. They assert that the killing of a blood relative, his mother of all people, is much worse than Clytemnestra’s murder of his father. Orestes's defence pretty much consists of calling Apollo for his defence Apollo says, "This (the murder of Clytemnestra) is justice." He goes on to tell the jury that the murder was Zeus's will and nobody should question Zeus. Orestes was merely doing what Zeus and himself encouraged him to do. This, of course, angers the Furies; it makes their arguments seem weak when it is at odds with such powerful Gods.