Why does Brutus go to war with Antony if he is going to kill himself?

First and foremost, Brutus did not plan to take his own life, but rather fell victim to the guilt of leading Rome into a civil war.  At the end of Shakespeare’s Julius Caesar, Brutus and Cassius wage war against Antony and Octavius with the end goal of retaining control over Rome.  In Act 5 Scene 1, Cassius asks Brutus if he is prepared to become a prisoner if they lose the impending war.  Brutus replies:

No, Cassius, no. Think not, thou noble Roman,

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.

And whether we shall meet again I know not.

Therefore our everlasting farewell take.

Forever and forever farewell, Cassius.

If we do meet again, why, we shall smile.

If not, why then this parting was well made (V. i. 112-120).

Therefore, Brutus plans to kill himself only if he does not win the war against Antony and Octavius.  He understands that if he is captured, he will be paraded in chains through the streets of Rome and executed as a public spectacle.  This alludes to another important element of Brutus’ decision to kill Caesar, to go to war with Antony, and to ultimately kill himself—his noble desire to act in the best interests of the Roman people.  Indeed, Brutus’ driving motivation to conspire against Caesar is to prevent Caesar from exerting absolute power over the Republic.  In his soliloquy at Caesar’s funeral, Brutus provides his reasons for his actions:

It must be by his death, and for my part

I know no personal cause to spurn at him

But for the general.  He would be crowned.  

How that might change his nature, there’s the question (II. i. 10-13). 

Brutus reveals his intentions were to protect the people of Rome from oppression by an authoritative dictatorship.  Yet, as the play progresses,  Brutus realizes the citizens of Rome are angered by his decision to stab Caesar, predominantly due to Antony’s moving rhetoric at the funeral.  Further, the assassination of Caesar has caused a faction in the government as two parties clamor for control:  Brutus v. Antony.  Thus, again believing he is acting in the best interests of the Romans, Brutus leads his soldiers to war against Antony at the battle in Philippi. 

As Brutus and Cassius meet Antony and Octavius on the battlefield in Philippi, the two armies exchange words that shed light on the reasons behind the feud.  Cassius states, “Antony, the posture of your blows are yet unknown. / But for your words, they robe the Hybla bees / And leave them honeyless” (V. i. 33-35).  Cassius uses a metaphor to accuse Antony of stealing from the bees and leaving them with nothing, which is a figurative comparison to the people of Rome.  Upon Caesar’s death, Antony changed Caesar’s will to get the assets that the people of Rome were to receive.  So not only does Brutus go to war with Antony to retain control of Rome, but to avenge the deception enacted by Antony in the wake of Caesar’s assassination. 

Finally, during the battle at Philippi, Brutus mistakenly senses a weakness in Octavius’ army and pushes his men in an effort to exploit the advantage.   However, Brutus proves overeager in his plan and loses the upper hand in the battle.   It is at this point that Brutus decides to kill himself, having let down the people of Rome.  He impales himself with his sword and cries, “Caesar, now be still. / I killed not thee with half so good a will” (V. v. 56-57).  Defeated, Brutus sacrifices himself with pride and dignity to allow Antony to gain control of Rome.