It is a simple equation really. God's = Immortality, Any God that does or can die,, is not a God. so,, If someone has killed Ares, then he really is not the God of anything. "Mortals can die. Gods cannot."
In the original mythology Ares' cousins, a pair of twin giants, sons of Poseidon named Otos and Ephialtes, once captured and locked Ares in a giant jar for about a year before he was rescued by the messenger-god Hermes. Apparently this imprisonment, presumably because he did not have access to any of the food and drink of the gods (or any form of nutrition, for that matter), nearly killed him. But otherwise, no, there's no ancient story about the death of this god.