Why does Macbeth speak of playing "the Roman fool"?
Macbeth seems to be referring to Brutus as "the Roman fool" because Brutus is famous in history for running onto his own sword when it appeared he and Cassius had lost the battle at Philippi to Antony and Octavius. Macbeth is displaying a more barbaric, less refined character. He sees no point in killing himself now that he has lost the battle with Malcolm and Macduff when there are thousands of enemy soldiers who would gladly do the job for him if given the opportunity. The most admirable thing about Macbeth is that he goes down fighting. His pertinent words in full are,
Why should I play the Roman fool and die
On mine own sword? Whiles I see lives, the gashes
Do better upon them. (Act V, Scene 8)
Macbeth has developed a totally negative attitude towards everything in life. When Seyton reports that Lady Macbeth has just died, Macbeth says,
She should have died hereafter;
There would have been a time for such a word.
(Act V, Scene 5)
The word "should" in the first line quoted above is in the subjunctive. Macbeth does not mean it would have been more convenient for him if his wife had died later; instead, he is saying everybody has to die sooner or later.
In Act V, Scene 3, he asks the Doctor disingenuously:
Canst thou not minister to a mind diseased,
Pluck from the memory a rooted sorrow,
Raze out the written troubles of the brain,
And with some sweet oblivious antidote
Cleanse the stuff'd bosom of that perilous stuff
Which weighs upon the heart?
The Doctor replies,
Therein the patient
Must minister to himself.
Macbeth reveals his contempt for medicine as well as for his own existence:
Throw physic to the dogs, I'll none of it.
Unlike Brutus, Macbeth no longer has any belief in anything. It would be quite out of character for Macbeth to kill himself with his own sword or in any other way, although it seems perfectly in character for the proud and image-conscious Brutus to kill himself by running onto his own sword at Philippi. Shakespeare may have toyed with the idea of having Macbeth commit suicide on the battlefield as a way of showing no man could compare with him in hand-to-hand fighting, but the playwright may have rejected the idea because it seemed out of character for semi-civilized Macbeth. Both Brutus and Macbeth, however, are motivated by the same pride. Brutus kills himself to prevent Antony and Octavius from triumphantly leading him through the streets of Rome. Macbeth does not want to fight Macduff when he learns that his enemy was "not of woman born" (the Apparition in IV.i says, "laugh to scorn/The power of man, for none of woman born/Shall harm Macbeth"). Instead, as Macduff says, "Macduff was from his mother's womb/Untimely ripp'd" (V.viii). Macduff threatens Macbeth with a fate similar to that which would have befallen Brutus if he had not committed suicide.
Then yield thee, coward,
And live to be the show and gaze o’ the time.
We'll have thee, as our rarer monsters are,
Painted upon a pole, and underwrit,
“Here may you see the tyrant.”
I will not yield,
To kiss the ground before young Malcolm's feet,
And to be baited with the rabble's curse.
Though Birnam Wood be come to Dunsinane,
And thou opposed, being of no woman born,
Yet I will try the last. Before my body
I throw my warlike shield! Lay on, Macduff,
And damn'd be him that first cries, “Hold, enough!”
(Act V, Scene 8)
Macbeth is demonstrating his resolve not to take his own life - to fall upon his own sword as a "Roman fool" would do out of despair. He is determined to fight to the bitter and bloody end. The link below will give you additional information about the character of Macbeth. Good luck!
Macbeth is alluding to the old Roman idea of the dignity of committing suicide. Obviously, he has considered suicide, but he decides that the Romans were fools to consider suicide to be a more dignified death than dying in battle.