The character flaws in both Macbeth and Lady Macbeth are established early in Act I. When Macbeth first hears the witches' prophecies, he is intrigued, wanting to hear more, but the idea of becoming King of Scotland seems totally outlandish to him. Once the first part of the prophecies is fulfilled, however, and Macbeth is named Thane of Cawdor, Macbeth's terrible ambition--his major character flaw--becomes apparent. When Duncan then names his son Malcolm, the Prince of Cumberland, his rightful heir, Macbeth's reaction shows that his ambition has taken root:
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
Macbeth's character flaw has been revealed clearly in this passage.
When Lady Macbeth receives news of these events in Macbeth's letter, her reaction shows that her character is as flawed as her husband's and in the same way. She immediately begins to plan their ascendancy to the throne:
Glamis thou art, and Cawdor, and shalt be
What thou art promised [Scotland's king] . . . .
Lady Macbeth's ambition knows no bounds as she begins at once to plan the murder of King Duncan so that Macbeth can claim the throne:
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements.
No sense of morality will deter Lady Macbeth from seeing her husband crowned.