Agamemnon was the most powerful king in Greece and the commander of the Greek forces in the Trojan War. The king of kings pops up in lots of myths, and most of the time he's a total jerk. (Check out the Iliad for lots of examples of Agamemnon's bad behavior during the Trojan War.) Overall, Agamemnon represents the power-hungry tyrant, who'll do anything to keep his power.
In this story, we see Agamemnon before the war kicks off. Guess what? He's a jerk then too. He sacrifices his daughter Iphigenia to appease Artemis, who's mad because he killed a deer in her sacred grove and bragged about it. The goddess refuses to let the wind fill the sails of the Greek fleet until she has Iphigenia, and the king totally obliges. If the fleet doesn't sail, Agamemnon will look like a total loser, so it's the chopping block for Iphigenia, even though the whole thing is her dear old dad's fault. How is that fair, you ask? Answer: it's not.
Eventually, however, Agamemnon gets what's coming to him. All that power he fought so hard to keep doesn't mean doodly-squat when he gets home to Mycenae. Instead of dying in battle, like a good warrior king should, Agamemnon gets stabbed to death in a bathtub at by his wife, Clytemnestra, who's been stewing on Iphigenia's murder for years. Although Clytemnestra is often shown as a villainess herself, she can also be seen an instrument of the gods' justice. She is the thing that finally punishes Agamemnon for his pride and lust for power.