Aegisthus is Clytemnestra's new lover. We met him at the end of Agamemnon, where he almost got into a fight with the Chorus of old men. Nobody seemed to like him then, and nobody likes him now either. Orestes is especially contemptuous of him, and thinks killing him will be a piece of cake.
Really, Aegisthus isn't onstage all that much, and when he is, he doesn't give any evidence that the public perception of him is off the mark. He proclaims his ability to see through any trickery at the very moment that he is being deceived and led to his death. Good work, genius. What does Clytemnestra see in this guy, anyway?