Book 18 allows readers to see more layers to the character of Achilles. He is not just a fierce warrior, but a man who loves in the truest sense of the word. His reaction to the death of Patroclus reveals his compassion and the depth of his emotions.
Hector, however, has sealed his fate. Achilles was not invested in this war, but with the death of his friend, Achilles has a reason to keep fighting. What was an unfortunate but commonplace consequence of war has become a step towards the downfall of the Trojans.
Achilles is the ultimate hero because, as we learn here, he does not fight for glory, but because of his compassion. It overwhelms him, and he goes on to avenge the murder of his closest friend. While revenge is not a modern theme, it was considered the duty of man in Greek times.