Shakespeare employs ghosts and apparitions frequently in his writings, especially of characters murdered or wronged by their deaths. These ghosts generally have a twofold purpose: to haunt the guilty conscience of their murderers and avenge their deaths, and to act as an ill omen foreshadowing some oncoming doom. Caesar's ghost in act IV is remarkably similar to Banquo's ghost in Macbeth. Both are the ghosts of men murdered by a close friend, who appear to bring about their murderer's end.
Though Brutus became the de facto leader of the conspirators to kill Caesar, he was not the one who originally planned to kill Caesar. Cassius spurned him to action, suggesting that justice could only continue in Rome with Caesar's death. Brutus agreed and helped carry out the deed, defending it to himself and to the Roman people with tales of Caesar's ambition and impropriety as a leader. Yet the appearance of the ghost implies that Brutus's conscience may not be entirely clear. Caesar was his friend, and though he believes his actions were necessary, he feels some guilt for having murdered a man who trusted him so. When the ghost appears, Brutus asks it to identify itself:
Brutus. Art thou some god, some angel, or some devil,
That makest my blood cold and my hair to stare?
Speak to me what thou art.
Ghost. Thy evil spirit, Brutus.
The ghost's response could be interpreted as simply the evil spirit that is haunting Brutus. However, "thy evil spirit" can also be read as the evil spirit of Brutus himself. Caesar's ghost represents the evil in Brutus' heart that revealed itself when he murdered Caesar. This evil is now come to haunt Brutus.
Caesar's ghost also appears to impart a cryptic message to Brutus:
Brutus. Why comest thou?
Ghost. To tell thee thou shalt see me at Philippi.
Brutus. Well; then I shall see thee again?
Ghost. Ay, at Philippi.
Brutus. Why, I will see thee at Philippi, then.
This plants the idea in Brutus's head that doom awaits him at Philippi. When he arrives there and the battle has turned against him, he decides to end his own life, laying the blame for his death with Caesar's ghost:
O Julius Caesar, thou art mighty yet!
Thy spirit walks abroad and turns our swords
In our own proper entrails.
This fulfills the foreshadowing laid earlier in the play. When Antony discovers Caesar's body, he suggests that Caesar's spirit will have revenge, even in death:
And Caesar's spirit, ranging for revenge,
With Ate by his side come hot from hell,
Shall in these confines with a monarch's voice
Cry "Havoc," and let slip the dogs of war
Caesar's ghost has succeeded. His appearance has shaken Brutus's convictions, and the announcement that Philippi is the place where he and Brutus shall meet again drives Brutus to suicide at the end of the play as he accepts his fate as ordained by the spirits. Caesar's murderer has answered for his crimes.