I can see why amysor (in Answer #2) thinks it is surprising that Brutus kills himself. In the play Cassius asks Brutus what he intends to do if they lose the coming battle. He receives a reply that strongly indicates that Brutus disdains to commit suicide:
BRUTUS:Even by the rule of that philosophy
By which I did blame Cato for the death(110)
Which he did give himself: I know not how,
But I do find it cowardly and vile,
For fear of what might fall, so to prevent
The time of life: arming myself with patience
To stay the providence of some high powers(115)
That govern us below.
Then Cassius asks another question:
And 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.
Brutus seems to be saying that he will not commit suicide and that he will not allow himself to be taken prisoner. Evidently amysor believes that Brutus chose to run onto his sword because he did not consider that to be suicide--or perhaps only a half-suicide, with Strato sharing the other half of the responsibility.
Frankly, this idea never occurred to me. But it seems plausible. Even ingenious.
Shakespeare relied heavily on Plutarch's "Life of Brutus" for his historical information. Here is what Plutarch has to say about the matter under discussion here.
"When I was a young man, Cassius, and without experience of the world, I was led, I know not how, to speak too rashly for a philosopher. I blamed Cato for making away with himself, on the ground that it was impious and unmanly to yield to one's evil genius, not accepting fearlessly whatever befalls, but running away. In my present fortunes, however, I am become of a different mind; and if God does not decide the present issue in our favour, I do not ask once more to put fresh hopes and preparations to the test, but I will go hence with words of praise for Fortune; on the Ides of March I gave my own life to my country, and since then, for her sake, I have lived another life of liberty and glory."
At least in Plutarch it seems pretty clear that Brutus was saying that he disapproved of suicide when he was young but that he has changed his mind about it as he has grown older and wiser through experience. Brutus doesn't seem to have any option but suicide.
It should not be surprising that Brutus kills himself at the Battle of Philippi in Shakespeare's Julius Caesar. In the first place, it is a matter of historical fact that Brutus actually did commit suicide as Shakespeare shows the incident in his play, which was by running onto his own sword. Shakespeare relied heavily on Plutarch's Life of Brutus as well as the Greek historian's Life of Julius Caesar and Life of Antony, and Shakespeare would not have wanted to rewrite history--although he did abridge it for dramatic purposes. Further, it does not seem likely that Shakespeare wanted Brutus's suicide to come as a surprise to his audience since he prepares them for it in an earlier scene. In Act V, Scene 1, Cassius asks Brutus:
Then, if we lose this battle,
You are contented to be led in triumph
Through the streets of Rome?
To which Brutus replies:
No, Cassius, no. Think not, thou noble Roman,
That ever Brutus will go bound to Rome;
He bears too great a mind.
Brutus and Cassius have been waging war against Antony and Octavius for over two years. Brutus knows he has no chance of receiving lenient treatment from his bitter enemies. He was the leader of the conspiracy against Julius Caesar and was the last person to strike him with his dagger. If captured alive he would be led in triumph through the streets of Rome and then undoubtedly would be executed. Antony likes him personally but would not allow that to influence him. Octavius has proven himself to be a ruthless and cruel man in the short time he had held power. He would certainly want to punish Brutus severly for his part in killing his uncle.
The situation is somewhat similar to the finale of Shakespeare's Antony and Cleopatra. Octavius has defeated Antony in Egypt and is holding Cleopatra prisoner. He is treating her with extreme kindness, but she suspects that he intends to take her back to Rome and lead her in a triumphal march down the major thoroughfare. When Cleopatra realizes that her suspicions are correct, she commits suicide by letting herself be bitten by two poisonous snakes.
It is suprising that Brutus kills himself because he vowed that he would never be taken prisoner or kill himself. He want back on his word, but he justified his suicide by making one of his men hold the sword as he ran into it. It was still suicide, and went against what Brutus said, which is ironic.