In Act II of Julius Caesar, when Cassius says, "Mark Antony, so well beloved of Caesar, should outlive Caesar. We shall find of him a shrewd contriver," he is expressing concern about Mark Antony's potential influence and ability to maneuver politically after Caesar's assassination. The correct interpretation is:
Even if they kill Caesar, Antony will be just as difficult to control.
Cassius is suggesting that Antony, who is close to Caesar and well-liked by the people, could pose a significant threat to the conspirators' plans. He implies that even without Caesar, Antony's cleverness and loyalty to Caesar would make him a formidable adversary.