When the alarm is sounded after the discovery of King Duncan's body, Lady Macbeth appears, seemingly anxious to know what terrible thing has happened. Macduff replies:
O gentle lady,
'Tis not for you to hear what I can speak:
The repetition, in a woman's ear,
Would murder as it fell.
The passage shows strong dramatic irony. Macduff shows great concern for Lady Macbeth. He believes she, being a woman, is so fragile that merely hearing the news of the King's death--and the horrible circumstances of it--would likely kill her. However, the audience knows the truth about Lady Macbeth. It is she who planned Duncan's murder, manipulated Macbeth into participating, intended to kill Duncan herself, and mocked her husband even for being upset after he killed the King. To her, Duncan's blood is merely something to be washed away with a little water. The "gentle lady" is neither; she is cold and evil in ways the audience understands but that Macduff could not even imagine.