Macbeth's valor is seen through the eyes of others when initially described. The audience themselves only know of the great feats he has undertaken as a result of what others say. The first to detail such glory is the Sergeant who talks of Macbeth's prowess on the battlefield:
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandished steel,
Which smoked with bloody execution,
Like valor’s minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseamed him from the nave to th' chops,
And fixed his head upon our battlements.
Duncan responds with calling Macbeth "valiant" and later referring to Macbeth's deeds as "honorable." The impression that this casts upon the reader is to recognize Macbeth's bravery and honor. At the same time, reality is shown to be filtered through another's perception. Even in the acknowledgement of Macbeth's greatness, we are unable to ascertain what truth is, having to take it from another vantage point.
It is within this construction of glory and praise that the witches confront both Macbeth and Banquo. The witches speak to illuminate aspects of the future for both men. They do so with prophecies meant to entice them, and seek to create a sense of want in each. They accomplish this in Macbeth, as reflective in Banquo's response to his colleague: "Good sir, why do you start and seem to fear/ Things that do sound so fair?" It becomes clear that the witches' purpose in speaking has taken hold of Macbeth with his protest to the witches upon their departure:
Stay, you imperfect speakers, tell me more.
By Sinel’s death I know I am thane of Glamis.
But how of Cawdor? The thane of Cawdor lives,
A prosperous gentleman, and to be king
Stands not within the prospect of belief,
No more than to be Cawdor. Say from whence
You owe this strange intelligence, or why
Upon this blasted heath you stop our way
With such prophetic greeting. Speak, I charge you.
Macbeth wishes to "charge" them from a position of power when it has become evident that the witches have begun to exert power over him.
As Macbeth embraces the prophetic words of the witches, it becomes clear that Macbeth must commit to eliminating the second part of their vision. In targeting Banquo and Fleance, Macbeth makes clear that he must dominate all in his hopes to attain the power that he now sees as his. In speaking to Lady Macbeth in Act III, sc. 2, Macbeth suggests that "We have scorched the snake, not killed it./ She’ll close and be herself whilst our poor malice/ Remains in danger of her former tooth." This reflects why Macbeth wants Banquo murdered. He knows that the witches offered the prophecy that "Thou shalt get kings, though thou be none" to Banquo. It is also for this reason that Macbeth commissions both of their death ("There shall be done/ A deed of dreadful note") and represents why he reacts in the way he does to Fleance's escape. Macbeth understands that his "perfect" plan has gone astray: "There the grown serpent lies./ The worm that’s fled/ Hath nature that in time will venom breed;/ No teeth for th' present." Macbeth's anxious state foreshadows what is to come.