The Question and Answer section for Iliad is a great resource to ask questions, find answers, and discuss the novel.
I'd go with Achilles. Dealing with his rage is the central action of the epic; he sacrifices many of his allies to his pride, refusing to fight because of an insult to his honor. His movement from rage to grief and wrath and finally to recognition...
Apollo was responsible for the plague. He sent the plague to punish Agamemnon for claiming Chryseis as a prize of war. He wants her returned.
I'm not sure which story "vengeance" is related to. You will need to tell me which battle you are referring to.