Using the following lines from Act I and Act IV, explain how the Macbeth described in the beginning of the play is different from the Macbeth who is speaking these lines in Act IV.
But all's too weak;
For brave Macbeth,—well he deserves that name,—
Disdaining fortune, with his brandish'd steel,
Which smok'd with bloody execution,
Like valor's minion,Carv'd out his passage
Till he fac'd the slave;
And ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
O valiant cousin! worthy gentleman!
Time, thou anticipat'st my dread exploits:
The flighty purpose never is o'ertook
Unless the deed go with it: from this moment
The very firstlings of my heart shall be
The firstlings of my hand. And even now,
To crown my thoughts with acts, be it thought and done:
The castle of Macduff I will surprise;
Seize upon Fife; give to the edge o' the sword
His wife, his babes, and all unfortunate souls
That trace him in his line. No boasting like a fool;
This deed I'll do before this purpose cool:
But no more sights!—
Act I Macbeth is the very picture of honor and courage. He lives only to serve the king and do his duty as the king's most worthy general. He has no concern for his own safety (he "disdains fortune"), and if it means he has to wade through raging battle to find the Norwegian general and cut him open from his navel to his jaw, that's what it means. He doesn't speak of his own exploits, either; we hear about them from the soldier. Even when Duncan praises him and Banquo for their deeds, they reply that they only did their duty.
Cut to Act IV. This Macbeth has murdered the anointed king in his own home (a double sin), his guards (grooms), and had cutthroats murder his friend Banquo (and tried to murder Banquo's son). Because the witches (the weird sisters) have told him to beware Macduff (and because Macduff did not appear at his banquet--and no one says no to a king), he fears that Macduff knows too much.
In these lines, he hints that he second-guesses his moves, probably because of the guilt and self-doubt he struggles with. He's no longer the self-assured man who waded purposefully into battle without thought for his own life. Here, he's deciding to stop thinking about his acts and simply do them; thinking just slows him down. Now, as soon as he knows that he needs to do something to protect his throne, he will do it--and the next thing he must do is to murder Macduff's family. The man who was riddled with doubts and guilt about killing Duncan has become a calloused murderer.