Macbeth is relieved and feels secure, because he knows that all men are born of women and that forests cannot move. When he learns that Macduff has fled to England to join Malcolm, Macbeth orders that Macduff’s castle be seized and, most cruelly, that Lady Macduff and her children be murdered.
Macbeth, amazed that the witches’ prophecy has come true, asks Banquo if he hopes his children will be kings. Banquo replies that devils often tell half-truths in order to “win us to our harm” (1.3.121). Macbeth ignores his companions and speaks to himself, ruminating upon the possibility that he might one day be king.
Shakespeare uses Macbeth to show the terrible effects that ambition and guilt can have on a man who lacks strength of character. We may classify Macbeth as irrevocably evil, but his weak character separates him from Shakespeare’s great villains—Iago in Othello, Richard III in Richard III, Edmund in King Lear —who are all strong enough to conquer guilt and self-doubt.
The three witches prophesize that Macbeth will become Thane of Cawdor and King of Scotland, and that Banquo will have sons who are kings. Shortly after, Macbeth is indeed given the title Thane of Cawdor. Scene 4. Duncan welcomes Macbeth and Banquo and tells them that he plans to make Malcolm his heir to the throne.
Macbeth is a brave soldier and a powerful man, but he is not a virtuous one. He is easily tempted into murder to fulfill his ambitions to the throne, and once he commits his first crime and is crowned King of Scotland, he embarks on further atrocities with increasing ease.
Macbeth is a courageous Scottish general who is not naturally inclined to commit evil deeds, yet he deeply desires power and advancement. He kills Duncan against his better judgment and afterward stews in guilt and paranoia. Toward the end of the play, he descends into a kind of frantic, boastful madness.
Macbeth may start out as a good person, but once his ambition gets the better of him and he commits his first act of treachery, he becomes the play's primary source of evil. After he murders Duncan, Macbeth feels compelled to keep killing in order to cover up his first crime and maintain his grip on power.