In "The Iliad," usually attributed to Homer, Achilles is a great hero among the Achaeans. At the beginning of the epic, he refuses to help the Achaeans fight agains Troy because he is angry at the king. As it happens,he will not help fight until his best friend, Patroklos, is killed in battle.
The actual killer of Patroklos was the Trojan hero Hector. However, he is only able to kill Patroklos with divine help. What happens is that Apollo hits Patroklos over the head and knocks his helmet off. That leads to Hector being able to kill him.