Sons of Pelops, after whom the Peloponnese is called, Atreus and his brother Thyestes, were refugees when they came to the court King Eurystheus of Mycenae who had married their sister.
The brothers jealousy reached its height when Atreus discovered Thyestes was having an affair with his wife.
He killed the two children of Thyestes and served them up in a banquet.
After he had eaten an been informed Thyestes called a curse not upon Atreus but upon his children and his children's' children.
Thyestes had a surviving daughter named Pelopia with whom he (Thyestes) fornicated and by whom she conceived a son, Aigisthos.
When Aigisthos grew up he turned around and murdered the now King Atreus and placed his father-grand-father Thyestes on the throne of Mycenae. But he stupidly didn't kill Atreus' other son, Agamemnon, who seized power at the first opportunity. Plus if he had it would have ruined a perfectly good story.
Now Agamemnon, who no doubt suffered from an overabundance of testosterone and after all, came from a dysfunctional home, wasn't a totally bad guy. He was just cursed. As I am sure you all recall, at the advice of his imported Trojan seer, but against his own desires, he sacrificed his daughter Iphigenia, to obtain favorable winds for his war fleet against Troy. He did it for the greater good. Plus Iphigenia was a colicky child.
The fleet had been beached at its assembly point on the coast of Evvia for several months waiting for the winds to change and time was wasting. There were mouths to feed and squabbles to avoid between the armed to the teeth tribes of Greece.
The sacrifice of Iphigenia, which worked by the way, was the straw that broke his wife Klytemnestra's back, as when capturing her as booty, in a previous conquest, he had already dashed to death her first born by her previous husband whom he also killed before her eyes. While Agamemnon had been away Klytemnestra had given him horns by her unfaithfulness with, you guessed it: Aigisthos and upon his triumphant return, they both killed him in his bath. Agamemnon had appointed Aigisthos regent in his absence too.
Here she is, left, after just completing the deed, axe in hand! She had 8 years to look self righteous before Oresties and Electra, Agamemnon's children revenged themselves upon them both.