Brutus, although arguably presented by Shakespeare as a heroic character, is deeply flawed in ways that lead to the defeat of the conspiracy against Caesar and the ruin of the conspirators. He is idealistic and a poor judge of popular feeling, he is irresolute and uncertain, and he is a poor military leader.
Brutus' idealism shows in his belief that the drift towards centralized power in Rome can be checked by the assassination of a single man, Caesar (Act II, Scene 1). He trusts that the faults of Caesar's ambition will be self-evident to the mob, and so not only fails to kill Mark Antony but even allows him to speak at Caesar's funeral (Act III, Scene 1). The result is that Antony's eloquence overwhelms Brutus' more intellectual approach and the conspirators find themselves being hunted by a furious mob rather than taking command of the direction of the state.
Brutus' irresolution and uncertainty similarly show themselves in the frequent actions he takes that undermine his ostensible goals. He has trouble making up his mind that killing Caesar is a good idea, and compromises to himself by trying to turn the assassination into something noble, as if a murder could ever be such (Act II, Scene 1; Act III, Scene 1). Later, in the field with Cassius, he starts a petty and pointless quarrel with him over a minor point, only to confess later that he has been distracted by the news of the death of his wife (Act IV, Scene 3). In a sense, the ghost of Caesar that appears to him is no more than the tangible form of his own worries and doubts.
Finally, Brutus is a poor military leader. At Philippi, he impulsively orders his troops forward to attack too early (Act V, Scene 3), seemingly wishing to just get the battle over with. The result is the defeat of the conspirators and his own suicide.