There are several reasons for Brutus to kill himself at the end of Shakespeare's Julius Caesar. He has been defeated in battle by the enemy, who will treat him brutally if he survives, probably making him the center of their triumph. He has seen the ghost of Caesar and believes that this is a sign from fate that his time has come. He asks both Clitus and Dardanius to kill him, but both refuse. Volumnius then refuses even to hold a sword out for Brutus to run against. Strato finally agrees to do this, and Brutus finally runs onto his sword, telling the ghost of Caesar with his dying breath that he now kills himself with a better will than he killed Caesar. In doing so, he feels that he has secured a worthy death, if not a glorious one.

Brutus, like most aristocratic Romans, was heavily influenced by Stoic philosophy, and Shakespeare depicts him as having a classically high-minded Stoic viewpoint throughout the play. For a Stoic, suicide was not a disgraceful death, but a noble one, since one remained in control of oneself to the very end of life, rather than surrendering to external forces. Brutus is therefore conscious of dying as he has lived—as a noble Roman—by choosing the most honorable way to die. The generous eulogy of his enemy, Antony, confirms that his memory will be honored, and that nothing in the manner of his death mars his reputation as "the noblest Roman of them all."

Though Brutus is undoubtedly a man of many talents, inspiring military generalship isn't one of them. His enormous tactical blunder at the Battle if Philippi is largely responsible for the defeat of Republican forces. If the Roman Republic weren't already dead, then it certainly is now.

Under the circumstances, things are looking pretty bleak for Brutus. As one of the leading figures in the assassination of Julius Caesar, it's a certainty that if he's captured alive, he'll be executed—but not before being paraded through the streets of Rome in chains as a captured felon. For someone of Brutus's sensibilities, and for a proud man with a heightened sense of dignity and self-worth, this in itself would be a fate worse than death.

And so, in order to avoid shame, public humiliation, and death, Brutus does what he considers to be the decent thing and commits suicide. As a Roman aristocrat, Brutus believes that this is a more honorable death than being executed by one's enemies.

This was a widespread attitude among people of his class, and so there's nothing remotely unusual about Brutus falling on his sword like this. Far from being regarded as a coward, Brutus will actually be widely commended for having taken his own life, even among his enemies.

Brutus' driving force throughout the entire play was his love for Rome and his  desire to better it. Toward the end of the play, Brutus comes to realize that the citizens of Rome are unhappy with his decision to kill Caesar, mostly due to Antony's convincing speech. Brutus has lost sight of his mission, and sees others dying and rioting because of his choices. Brutus decides to "give the people what they want" and take himself out. At this point, he truly believes that Antony is the better leader, and so he sacrifices himself to end the battle.

In Act V, Scene 1 of Julius Caesar, Cassius asks Brutus what he intends to do if they lose the impending battle with Antony and Octavius.

You are contented to be led in triumph
Thorough the streets of Rome?

To which Brutus replies:

No, Cassius, no. Think not, thou noble Roman,(120)
That ever Brutus will go bound to Rome;
He bears too great a mind. But this same day
Must end that work the ides of March begun.

Evidently both Cassius and Brutus realize that if they were captured alive they would be taken back to Rome, led in triumph down the main thoroughfare, and then executed. Since it was a matter of historical fact, according to Plutarch, that both men committed suicide, Shakespeare had little choice but to have them do so in his play. All four principals came to bad ends. Brutus and Cassius killed themselves. Antony also killed himself in Egypt after being defeated by Octavius. Octavius became an emperor and a god, but he ended up being poisoned by his own wife.

Octavius and his successors, including the notorious Caligula and Nero, are written about in the excellent novel I, Claudius by Robert Graves and its sequel Claudius the God. These books were adapted by BBC to a long television series titled I, Claudius, in which many of Great Britain's finest actors and actresses appeared, including Derek Jacobi as Claudius. This series is available on DVD. 

At the end of Julius Caesar, Brutus observes the ruination of his country through civil war.  He had joined in a conspiracy to assassinate Caesar because he allowed Cassius to convince him that his friend Julius Caesar would be the ruin of Rome.  Because he loved his country more than he valued his friendships, he joined Cassius and the other conspirators in the plot.  After Caesar's death, instead of Rome benefitting, it was thown into civil war once Mark Antony got a chance to sway the crowds against the conspirators in his funeral oration.  That, too, was Brutus' fault because he underestimated Mark Antony and instead of refuisng to let him speak, as Cassius wanted, Brutus argued that allowing Antony before the crowd would be a good thing to lend sympathy to their cause.  How wrong he was.  Antony turned the crown against the conspirators and began a riot that led to war.  Too late, Brutus realizes the error of his ways and decides that his punishment must be death.  He carried out hiw own death sentence by running on his sword, and that it what led Mark Antony to call him "the noblest Roman of them all".

Why does Brutus choose to commit suicide at Philippi in Julius Caesar?

Choice A is the correct answer: To ancient Romans, suicide was an honorable way to die if it helped one avoid defeat and humiliation.    

 In Ancient Rome, suicide was considered an honorable alternative to being humiliated by the enemy.  Brutus and Cassius did not want to be captured by Antony and Octavius and marched in triumph through Rome.  A triumph was a military parade where the victor showed off his spoils and his captives.  It would have been the ultimate defeat for Brutus and Cassius. 

Before the battle, Brutus and Cassius discuss their plans if things go wrong.  They do not plan to be Antony’s captives.  This is a civil war.  To be marched in their own city as captives is unthinkable. 

CASSIUS

Then, if we lose this battle,
You are contented to be led in triumph
Thorough the streets of Rome?

BRUTUS

No, Cassius, no: think not, thou noble Roman,
That ever Brutus will go bound to Rome;
He bears too great a mind.  … (Act 5, Scene 1) 

What do you do then?  If victory is out of grasp and it looks like there is a certain defeat, Brutus and Cassius agree that they will kill themselves.  In fact, each of them do.  Cassius thinks that he sees his enemies surrounding his men, when in fact it is a victory celebration he sees. He commits suicide prematurely, on his birthday. 

Brutus commits suicide later, when he knows that all is lost. 

… Countrymen,
My heart doth joy that yet in all my life
I found no man but he was true to me.
I shall have glory by this losing day
More than Octavius and Mark Antony
By this vile conquest shall attain unto.
So fare you well at once; for Brutus' tongue
Hath almost ended his life's history … (Act 5, Scene 5) 

Antony later calls Brutus the noblest Roman of them all.  He believes that unlike some of the other members of the conspiracy, Brutus was not involved for fame or power. He really was doing what he thought was best for Rome.

In the play Julius Caesar by William Shakespeare, why does Brutus think he must commit suicide?

In Act V, Brutus and Cassius quarrel; Cassius thinks that their troops should rest and wait for their enemy to march upon them while Brutus insists that they engage the enemy at Philippi.  However, this taking  battle early with the troops of Octavius, Lepidus, and Marc Antony, the triumvirate, proves to be a tactical error, and the troops of Brutus and Cassius are defeated.  Brutus calls together the few survivors, and tells them to rest; he then asks Clitus to kill him, but his servant replies, "Iill rather kill myself."  As Brutus speaks to himself, Dardanius and Clitus overhear him say,

 Why, this, Volumnius:
The ghost of Caesar hath appear'd to me(20)
Two several times by night; at Sardis once,
And this last night here in Philippi fields.
I know my hour is come.

Brutus is defeated and bids farewell to the others and expresses his great regret that his final hour is so. Finally, Brutus retains Strato to assist him in his suicide, for he would rather die than be put in bondage or surrender.

When Brutus is defeated and dies, Antony and Octavius arrive to eulogize him.  Marc Antony calls him "the noblest Roman of them all while Octavius Caesar issues the order to give Caesar the "respect and rites of burial."