Cassandra is the guide and mentor who could have been, but wasn't. Because she is able to see into the future, there might have been a chance that she could have persuaded Agamemnon to be on his guard; perhaps she could have saved him from his fate. Or, then again, maybe not, since Cassandra had been condemned by Apollo to prophecy and never be believed. In any case, right before she goes to her death, Cassandra seems pretty happy that Agamemnon is going to be killed, because that will mean that her Trojan countrymen have been avenged. At the moment, we're 3 for 3: Agamemnon's a weird protagonist, Clytemnestra's a weird antagonist, and Cassandra is one heck of a weird guide and mentor.