|On the battlefield itself, there is no good or evil- only those who survive|
While we know Achilles has some very negative characteristics- his overweening pride and his elevation of his glory before the livelihood of his own comrades, we are shown that he has good characteristics to him as well- his valor and courage on the battlefield, and the deep bond that he shares with his friends- Patroclus most of all. He does have the ability to realize that he made some terrible mistakes throughout the poem, and the guilt of owning up to them is part of what makes the scene between him and Priam at the end so powerful.
Hector, far and away the most noble of all the major characters to fight in the conflict, I have mentioned before, also has his dark side. He at times revels in the slaying of Troy's enemies and disregards the portents of the gods at times. Hector is probably the clearest example of how war can turn even noble men into savage beasts when the time comes.
It is not only the main characters that are blurred in their motivations and agency. The two warring factions are also hard to judge as to whether they should be put in the classically 'good' or 'evil' camp.
While the Greeks are motivated by revenge and greed, they also legitimately want justice to be served for the outrage Paris committed when he stole Menelaus' wife and a horde of priceless treasures. The Trojans, while wanting to defend their country, are themselves enablers of Paris' crimes by refusing to give him up.
Most brilliantly, the focus of the actual action is not so much on Greeks and Trojans- but human beings. Abstract ideals and morality is crushed, almost literally, in the clash between the two armies. When people die, they are painted in their full humanity just as it is being snatched away from them. Greek and Trojan labels are immediately cut away as men are being cut down. Sword, spear, and arrow don't care about nationality or status in society. A Greek casualty can easily be a Trojan casualty and vice versa. The lines between combating sides is erased utterly in actual combat.
This dynamic prevents the Trojan War story from devolving into a simplistic, one-dimensional clash between good and evil, and may be the most enduring reason why it is the standard for any great war story. I try to the best of my ability to follow this model in my own work, so that the two sides and their characters are fleshed out and easy to become enthralled by.