The greatest aspect of Agamemnon's guilt is that of his family's guilt. (From House of Atreus)
The god-defying descendants of Tantalus committed unspeakable crimes that cried out for revenge, ultimately turning brother against brother, father against son, father against daughter and son against mother.
It began with Tantalus who served his son Pelops as a meal to the gods to test their omniscience. Demeter alone failed the test and so, when Pelops was restored to life, he had to make do with an ivory shoulder.
When it came time for Pelops to marry, he chose Hippodamia, the daughter of Oenomaus, king of Pisa. Unfortunately, the king lusted after his own daughter and contrived to murder all her more appropriate suitors during a race that he had fixed. Pelops had to win this race to Mount Olympus in order to win his bride, and he did by loosening the lynchpins in Oenomaus' chariot, thereby killing his would-be father-in-law.
Pelops and Hippodamia had two sons, Thyestes and Atreus, who murdered an illegitimate son of Pelops to please their mother. Then they went into exile in Mycenae, where their brother-in-law held the throne. When he died, Atreus finagled control of the kingdom, but Thyestes seduced Atreus' wife, Aerope, and stole Atreus' golden fleece. As a result Thyestes once again went into exile.
Believing that he had been forgiven by his brother Thyestes eventually he returned and dined at the meal his brother had provided him. When the final course was brought in, the identity of Thyestes' meal was revealed, for the platter contained the heads of all his children except the infant, Aegisthus. Thyestes cursed his brother and fled.
Agamemnon's fate is directly linked with his violent family past. His death appears to be the result of several different patterns of revenge. Upon his death, Clytemnestra does remark that she hopes that "the thrice gorged demon of the family" can be appeased.
As the ruler of all of Argos and husband to the duplicitous Clytemnestra, Agamemnon is highly a complicated character and it is very difficult to distinguish whether he is virtuous or immoral. There are many multi-facets of Agamemnon as a character. At times he is depicted as being very moral, and at other times, completely immoral. Although his presence in the play is very brief, his actions are the roots and the reasons for much of the conflict in all three plays of the trilogy. Not only that, but Agamemnon's hopeless dilemma to seek vengeance through the use of violence sets the stage for much of the dilemmas yet to come in the trilogy, thereby making Agamemnon an essential character in Oresteia.
Due to Agamemnon's sacrifice of his daughter for the sake of ambition and the curse of the House of Atreus, both crimes ignite a spark in the Oresteia that compels the characters to seek a revenge that has no end. Both crimes seem to indicate Agamemnon's guilt, some of it as a result of his own actions but conversely another portion of his guilt is that of his father's and his ancestors. One could argue that had not Agamemnon and Atreus sparked the initial flame to the curses, this vicious cycle would have been less likely to occur and such bloodshed would have not transpired. However, it seems from the Oresteia that these brutal murderous actions were required as some form of blood sacrifice to appease divine anger with the house of Atreus. When one reaches the close of the trilogy it appears that the hunger of "the thrice gorged demon" has finally been satisfied.