Macbeth's actions are an example of dramatic irony because the audience is well aware that Macbeth is responsible for the murder of King Duncan. However, Macbeth's actions and words in Act II, Scene III are significant for a number of other reasons as well. Firstly, the act of killing the guards ensures that the guards would never sober up and inform any of the lords that Lady Macbeth was responsible for their inebriation. While the guards would still probably be thought guilty of killing Duncan, the opportunity for them to tell their side of the story could potentially plant doubt within the minds of the others, and therefore possibly begin to point the finger at the Macbeths. The killing of the guards marks the first instance in which Macbeth takes measures to destroy additional potential threats to his safety. He continues this pattern throughout the play by hiring murders to kill Banquo and Fleance, and later going after Macduff and his family.
Further, Macbeth's words, and Lady Macbeth's reaction to them, reveal that he isn't very cunning, which reinforces Lady Macbeth's preexisting concerns. At this point in the play, Lady Macbeth has already warned him of his inability to hide his thoughts when under pressure. The tension of the scene increases as Macbeth is forced to explain his actions to Macduff and the others. He says that seeing the murderers asleep "[s]teeped in the colors of their trade" so near the body of Duncan, the murder of whom constitutes a crime against nature, was too much for him. He then asks "[w]ho could refrain/that had a heard to love, and in that heart/ Courage to make 's love known?" (2.3, 134-137). In other words, he loved Duncan so much, that he did what anybody would do in that moment. How could anybody blame him?
This scene is further enhanced by Lady Macbeth's reaction to Macbeth's attempts to explain himself. Upon seeing the focus turning toward her husband, she feigns shock and pretends to swoon to draw attention away from Macbeth. This also functions as dramatic irony, as the audience knows why she is pretending to swoon. As previously stated, it also reinforces the fact that she doesn't trust Macbeth to be able to get himself out of the situation, and she has good reason to be concerned. As the play progresses, Macbeth develops the habit of getting himself in over his head, time and time again.