Address Brutus's leadership abilities.

Overall, one could argue that Brutus is temperamentally unsuited to a position of leadership. For one thing, he's far too noble, too unselfish to survive for very long in such a treacherous, cutthroat environment. Brutus is an idealist; he loves the Roman Republic more than anything else. In some respects, this is a very important quality in a leader. But the problem is that the other assassins aren't motivated by the same noble aims as this most honorable of Romans. Brutus kills Caesar because he wants to save Rome from tyranny; his coconspirators stick the knife in—literally—for purely personal gain. Brutus actually believes in something, believes in a higher cause, and for a potential leader of Rome that's a fatal handicap.
Brutus's leadership qualities—or lack of them—are much in evidence on the field of battle. While the much more grounded Cassius understands the necessity of adopting defensive tactics in relation to Mark Antony's forces, Brutus foolishly decides to take the fight to the enemy at Philippi, with catastrophic results. Brutus has had his head well and truly turned by the responsibilities that military leadership entails. Instead of dealing with the practicalities of defeating the enemy, he's become enamored of his own legend, seeing himself as playing a lead role in a gigantic cosmic drama. It is his destiny to engage with the enemy at Philippi, he thinks.
Ironically, Brutus, who sacrifices his dear friend for the greater good of Rome, has now sacrificed the cherished cause of Roman republicanism to fulfill his vague, ill-defined sense of destiny. For that reason more than any other, his abilities as a leader must be called into serious question.