If Agamemnon is the protagonist, then Clytemnestra, who deceives him and murders him, is definitely the antagonist. That said, it is striking that Clytemnestra seems much more active than any other character in the play. Thus, in a weird way, you could say that she is its protagonist too. Think about it: once we learn that Agamemnon is coming home, Clytemnestra is the one who gets the ball rolling for everything else. She is the one who orders sacrifices to be made throughout the city to celebrate Agamemnon's return, she is the one who welcomes him and convinces him to walk on the purple fabrics, and she is the one who kills him and Cassandra. At the end of the play, she is the one who defuses the conflict between Aegisthus and the Chorus, and who suggests that she and Aegisthus will hold joint rulership in Argos. Clytemnestra's dynamic role at all of these important moments makes her a very unusual antagonist.