Another way of looking at the theme is through power and ambition. Brutus decides to murder his dear friend just in case he is to become corrupt. The power Caesar is able to wield is incredible. This worries the senators, particularly Brutus, who states he would sacrifice his own life for the good of Rome. Ironically, though, Caesar really hasn't shown any hints that he will be corrupted by his power. How "est qui tu brutus" ties into this theme is how Caesar feels betrayed that even his close friend Brutus would conspire to kill him when it is really unwarranted. They are just killing him in case he becomes corrupt. So much for free will.
Another interesting side to this is, of course, Cassius, who manipulates Brutus into taking part in the scheme to kill Caesar. While Brutus is the most noble of characters, he alone is acting for the good of Rome, Cassius is greedy and tricks Brutus into helping bring Caesar down when Cassius wants the same ambition and power that resulted in Caesar's murder.