We are told that before the war, Achilles had been a man of great love (philotes), but certain events during the war had darkened his soul:
‘He had made savage the high-hearted spirit within his body.’
His anger is the main theme of the Iliad. In fact, the first word of the Iliad is wrath (mêmis). The word ‘mêmis’ was usually used only in reference to the gods, suggesting the superhuman nature of his anger.
His wrath was first unleashed when Agamemnon took his concubine Briseis. Achilles had acquired princess Briseis when sacking a town near Troy. She had become an important part of his timê, but the two also became fond of each other. Achilles even likened their relationship to that of a husband and wife. But when Agamemnon lost his own concubine Cryseis, he took Briseis as compensation for his loss. Having his timê violated, Achilles declared in anger that he and his men (the Myrmidons) would retreat from the war and return home.
"The Great World History Book"