Equivocation is an important theme in Macbeth. The first time it's mentioned is in a humorous context in the Porter scene, where the Porter welcomes to his imagined hell "an equivocator who could swear in both the scales against either scale." In other words, an equivocator is able to avoid justice by speaking falsely, while seeming honest. Later in the same scene, scene 3 of act 2, we see Macbeth himself become an equivocator when he talks about his rage at Duncan's murderers. Macbeth also equivocates when he appears to miss Banquo so deeply at the feast, even though he's just had him killed. The tyrant, in turn, becomes the victim of equivocation when the Witches tell him that no man of woman born can kill him and he will not be vanquished until Birnam Wood come to Dunsinane. This is where irony comes in. These pronouncements are technically true, as MacDuff was born by Caesarean section and, before Macbeth dies, Birnam Wood does come toward Dunsinane in the form of branches carried by soldiers. Macbeth calls this "th' equivocation of the fiend / That lies like truth," and ultimately, equivocation undoes him. He feels secure in his belief that he could trust the Witches' prophecies, based on his previous experience with them, but he is wrong, which leads to his death.
As Shakespeare wrote Macbeth, equivocation leads directly to Macbeth's demise. Irony is used more to reveal theme and provide unity; a dominant idea in the play contains irony.
The witches use equivocation to fool Macbeth. They tell him that no man born of woman can harm him, and that he will be safe until Birnam Wood rises up to attack him. This leads Macbeth to think himself invincible and gives him a false sense of self-confidence. Macbeth is set up to believe these equivocations when the witches' first predictions seemingly come true: Macbeth is named Thane of Cawdor and he does become king. This leads him to replace his wife as adviser with the witches. Whatever else she is, Lady Macbeth is a loyal wife and has Macbeth's best interests in mind. The witches do not.
A central idea in the play involves irony: what's fair is foul and foul is fair. Few characters are what they are supposed to be or what they seem. Cawdor and Macbeth seem like loyal thanes, but they are not. Macbeth and Lady Macbeth undergo gender role reversals leading up to the assassination of Duncan. Macbeth is supposed to be Banquo's friend, but orders his murder.
Irony isn't something that necessarily leads to Macbeth's demise, as equivocation is, but it demonstrates how unnatural Scotland's political scene is in the play.
Equivocation: A statement that is not literally false, but is designed to mislead. Macbeth is led to his destruction by the equivocation of the witches, particularly as it is demonstrated through the appearance of the three apparitions in Act IV, scene i. Macbeth is told to "Beward Macduff!" Then he is told that "none of woman born" can harm him. Finally, he is given this reassurance:
Macbeth shall never vanquished be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him.
The statements prove to be true, literally, as final events in the drama unfold, but they mislead Macbeth, as intended, into a false sense of invulnerability. As a result, he continues in his wicked ways and is beheaded by Macduff.
Before his death, Macbeth realizes the witches have equivocated. Macduff's mother had not born him naturally at his birth, and Birnam Wood does move up the hill, as soldiers cut down boughs to shield themselves in their attack. Ironically, by feeling safe, Macbeth is set upon a straight path to his destruction. Further irony can be found in this; in one of the three prophecies, the witches did not equivocate, but Macbeth did not recognize the truth when it was presented to him. "Beware Macduff!" was not misleading at all.