Achilles remained by the black ships, raging. / He never appeared at the place of assembly, never / went into battle, but gnawed on his heart as he sat there. / And he longed for the war: for the battle shouts and the fighting.

This quote makes a concise summary of Achilles’s state of mind during most of The Iliad. His anger keeps him away from the battlefield, even though he sorely wants to be a part of the war. While Achilles stays in his tent, his anger festers. Because he cannot direct that anger to Agamemnon, he eventually turns it on Hector after he kills Patroclus. 

I hate like the gates of Hades / the man who says one thing and hides another inside him. / So, when I speak, I will say what is on my mind. / I will never be brought around by Lord Agamemnon / or by anyone else.

Part of Achilles’s problem with Agamemnon is that he feels the king cheated him and did not act honorably. It is customary for the Achaean army to split up the spoils of war evenly, but, in taking Briseis, Agamemnon robbed Achilles of his earned prize for fighting. As a result, when Odysseus and the rest of the envoy come to persuade him to rejoin the battle, Achilles prickles at the hidden question. Achilles wants Agamemnon to be transparent about his plea, and to give Briseis back to him. Achilles is not interested in being charmed into forgiveness. Neither does he maintain a façade of politeness about his feelings toward Agamemnon. Achilles holds tightly to his grudge and makes certain everyone knows how much he dislikes the king.

Achilles’s bravery, far / from profiting others, profits only himself, / and soon, I think, he will shed bitter tears, when his comrades / have been cut to pieces and it is too late to save them.

Nestor makes an acute observation of Achilles’s motivations. Although the epic calls Achilles a hero, the title does not carry the same connotation of moral goodness that we commonly use today. Instead, “hero” refers to a person who achieves immortality and glory on the battlefield. Nestor’s assessment of Achilles’s moral stance is that no amount of desperation or danger will make Achilles do something he doesn’t want to do. Ultimately, Achilles helps the Achaeans win the war, but his only real goal in fighting the Trojans after Patroclus’s death is revenge. It just so happens that his motivation lines up with what the Achaeans need, not the other way around.