Even for a guy who's got an entire tragic trilogy (the Oresteia) named after him, Orestes is one messed up dude. But hey, let's cut him some slack. He comes from a seriously dysfunctional family situation. Where to begin? Let's go way back. Orestes's grandfather, Atreus, butchered the children of his brother, Thyestes, and served them up to him for dinner. Then, Atreus's son, Agamemnon, sacrificed his own daughter, Iphigenia, while on his way to make war against the Trojans. While at Troy, he had affairs with various captive women; when he came home, he brought Cassandra, a Trojan princess, with him as a captive mistress. Then Clytemnestra, Orestes's mother and Agamemnon's wife, killed Agamemnon and Cassandra, and shacked up with Aegisthus, the last surviving child of Thyestes, which also makes him Orestes's cousin.
Orestes wasn't around when Agamemnon was killed, because Clytemnestra had already sent him away to be brought up in Phocian. Towards the end of Libation Bearers, Orestes accuses his mother of having sent him away specifically so that she could freely carry on her affair with Aegisthus, without having to worry about her son seeing her. So, basically, the dice were loaded against Orestes from birth: dysfunctional family with various members killing each other, a lack of positive male role models, and a mother who abandoned him. On a more positive note, though, Orestes seems to have had a very positive experience in his adopted family, and has become best friends with their son, Pylades.
So far so… bad. How the heck is Orestes going to deal with his family now that he's come to manhood. Well, Orestes becomes obsessed with the search for a father figure. Because his father is dead, the only way he can do this is by diverting all his energies into an obsessive quest for revenge. In the process, he neglects the seriously lowdown stuff his dad did. (When his mom complains about Agamemnon's affairs at the end of the play, Orestes simply treats that as acceptable male behavior.) Another problem with Orestes's quest for revenge is that it pits him against his own mother, but we'll come back to that.
The play begins with Orestes in full-on revenge mode. By dedicating locks of hair to the local river-god and to his father, Orestes shows his intention of being a dedicated, pious son and defender of his fatherland. He is welcoming towards his sister, and piously joins her and the Chorus in singing hymns to the gods and to the spirit of Agamemnon. When the time comes for putting the revenge plan into action, he takes charge of the situation by telling everybody else what to do. This fits in with the image he is going for – the strong, upstanding warrior-type.
So far so good. But then things start to go a little bit off the rails again once the plan gets put into action. First of all, it may be worth mentioning that the revenge plan is only accomplished through trickery: he and Pylades pretend to be travelers from Parnassus. Among other things, they claim to be delivering a message that Orestes is dead. This gains them access to the palace. This is a nifty plan and showcases Orestes's cleverness, but it also doesn't sit so well with his whole upstanding warrior persona – especially when he criticizes Clytemnestra and Aegisthus for using trickery against his father. Orestes's contradictions come to the forefront when he finally confronts his mother. He shows off his intelligence and willpower when he goes head-to-head with her in arguing over who's to blame for Agamemnon's death, and how much. In the process, he advances a strongly conservative, patriarchal view of gender relations.
Finally, Orestes drives Clytemnestra offstage and kills her. When he comes back onstage, however, he shows that all the willpower in the world can't make it an easy thing to kill one's own mother. In fact, he even starts doubting whether his mother really murdered Agamemnon. When Orestes tells us that he sees the horrible Furies of vengeance coming after him, it isn't clear if they're really there or if he's just off his rocker. Either way, when Orestes runs off to Delphi to be purified, it's clear that the contradictions in himself – and especially in his situation – have gotten the better of him. In many he ways, he is much worse off now than at the beginning of the play.