Considering how so much of what is known about Achilles is relayed through Homer’s epic poem the Iliad, this poem might be a good ancient source to use in an argument about whether Achilles is good or bad.
Using Homer’s ancient poem, it’s possible to argue that Achilles is good. Although, there’s probably a better, nuanced way to communicate Achilles’s hypothetical goodness. It could be more articulate to say that, in the Iliad, Achilles demonstrates qualities that are typically praised by society. This traits include courage and loyalty. Achilles bravely takes part in numerous battles and demonstrates his allegiance to his best friend, Patroclus, by confronting those who killed him.
But by citing Homer’s ancient text, it’s also possible to argue that Achilles is bad or demonstrates qualities that most people in a given society would not overtly praise or esteem. Achilles’s desecration of Hector’s corpse is not commendable. More so, some might argue that Achilles it too willing to wage war, and thus his violence is more bloodthirsty than valiant.
A contemporary writer, Sam Jordison, describes Achilles as “a killer, arguably a rapist, and certainly a pillager.” In conversation with this modern source, it’s not hard to argue that Achilles is bad. It shouldn’t be too difficult to find one more modern source that portrays Achilles as vengeful and predatory.
As for how Achilles’s goodness or badness enhances an understanding of the ancient world—it could make the ancient world seem less strange or unfamiliar. As with contemporary society, the ancient world gravitated toward heroes and reducing people to good or bad instead of thinking about how people are rarely all good or all bad but a little bit of both.