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So we all know that the Iliad is about the anger of Achilleus, but what we sometimes forget is that it takes two to tango. Even though Achilleus's reactions to things are typically over-the-top , it's obvious that Agamemnon treated him unfairly. That's because Agamemnon, at heart, is little more than a great big bully.
How else does one interpret the fact that Agamemnon repeatedly refuses to give up the girl Chryseis, even though hundreds of his soldiers are dying from the plague? When he then insists on stealing Achilleus's girlfriend, he doesn't have any good reason for it except making sure his "honor" remains intact—and to punish the Myrmidon warrior for challenging him in public. When he later reveals that he never slept with Briseis, it seems less a reflection of any decency on Agamemnon's part than a reminder that he was never really interested in her... all he wanted was to throw his weight around.
To make matters worse, Agamemnon repeatedly shows himself to be an incompetent and generally irresponsible commander. His incompetence comes forward in his pointless decision to test the loyalty of his soldiers by telling him they can head home. This, of course, simply provokes a mass exodus for the ships, which is only stopped by the tag-team of Odysseus and the goddess Athene.
Ironically, heading for home seems to be frequently on Agamemnon's mind. This is exactly what he suggests doing the night the Trojans camp on the plain, and then again the next day, in the middle of a battle. On both occasions, he is reminded that this suggestion is the ultimate insult to the men who have sacrificed so much on his and his brother Menelaos's behalf.
He also has a not-so-charming habit of blaming the gods for pretty much everything (although, to be fair, they are a pretty meddlesome bunch). His go-to attitude is "Not my fault!":
(Agamemnon:) This is the word the Achaians have spoken often against me
and found fault with me in it, yet I am not responsible
but Zeus is, and Destiny, and Erinys the mist-walking
who in assembly caught my heart in savage delusion
on that day I myself stripped from him the prize of Achilleus
Yet what could I do? It is the god who accomplishes all things. (19.85-90)
Does this guy have any redeeming characteristics? Hard to say. He does claim, in Book 9 and then again in Book 19, that it was only madness that made him insult Achilleus so badly. On both occasions, he also makes extremely generous offers of gifts to Achilleus—including marriage with his own daughter.
But these claims and offers – especially the first time around – are colored by the fact that he desperately needs Achilleus's help. Agamemnon's most genuinely decent gesture may be his gracious acceptance when Achilleus gives him the prize for spear-throwing, sight unseen, especially when he then gives this prize to the herald, Talthybios.
All in all, though, Agamemnon's actions confirm Achilleus's insults from Book 1: he is greedy for power and wealth, but not willing to stick his neck out to earn them.