From reading "Julius Caesar," we can learn that "ambition should be made of sterner stuff" as Marc Antony affirms in his funeral oration in Act III. While Antony's remark is meant to be ironic as he seeks to persuade the Romans against Brutus and the other conspirators, it becomes rather prophetic for both Antony and Brutus and Cassius, who are each flawed:
1. Brutus is idealistic and, as such, is impractical at times. He fails to understand the logic behind Cassius's urging him to forbid Antony to speak and later to kill Antony. Instead he believes that if he offers his hand in friendship, Antony will take it:
Our hearts/ Of brothers' temper do receive you in/With all kind love, good thoughts, and reverence (III, i, 187-190)
Had Brutus been made of "sterner stuff," he would not have offered his friendship. Instead, he would have eliminated any competitors from the goal of ruling Rome.
Later, Brutus waivers in his decision making. He argues with Cassius about the battle at Phillippi in Act IV. When Cassius urges Brutus to remain where they are and let their enemies come to them, Brutus argues that there is "a tide in men's lives" and they must seize the opportunity. This advancement of his army is a fatal mistake for Brutus as well as for Cassius, who gives in to Brutus.
2. Antony, who accuses the conspirators of cruelty and dishonorable, becomes brutal himself as his ambitions increase. In Act IV, Octavius, Caesar's nephew and part of the triumvirate who become crueler than Caesar ever has been, speaks with Antony. Antony tells Octavius that Lepidus is unfit to have so much power, so he will use Lepidus to achieve his political objective and then have him killed. Also, he will allow his nephew, Publius, to die.
These many then shall die, their names are prick'd...He shall not live; look, with a spot I damn him. (IV,i,2-7)
Antony, the loving friend of Caesar, becomes corrupted by ambition and its accompanying power.
3. Cassius, who is flawed by his jealousy, becomes corrupt in his ambition as well. He argues with Brutus frequently, threatening him physically and, later, threatens suicide when Brutus accuses him of taking bribes. Because he loves Brutus, Cassius gives in to him against so he can retain his friendship. An ambition made of "sterner stuff" would not be concerned about friendship: "I that denied thee gold will give my heart/Strike as thou didst at Caesar, for I know/When thou didst hate him worst, thou lovedst him better/Than ever thou lovedst him better. Tragically for Cassius, his emotional acquiescing to Brutus against his logic costs him his defeat in battle and his death.
Ironically, that for which the conspirators have killed Caesar, is the cause of what destroys them: ambition.