Macbeth has several character flaws that combine together into a "perfect storm" that causes his downfall and tears apart his country.

In act 1, soon after the witches depart, Macbeth learns that he is now Thane of Cawdor, as they had predicted. He wonders if the other part of the prophecy, to be king, will come true. While part of him wants to believe, he is gripped by fear.

...why do I yield to that suggestion

Whose horrid image doth unfix my hair

And make my seated heart knock at my ribs,

Against the use of nature? Present fears

Are less than horrible imaginings....

Aware that fear is holding him back, Macbeth has an internal conflict with his other flaw, ambition. After speaking with the king, he begins to realize that he is taking seriously the idea of usurping his role, and worries about the black desire of his own ambition.

Stars, hide your fires;

Let not light see my black and deep desires:

The eye wink at the hand; yet let that be,

Which the eye fears, when it is done, to see.

By scene 7, after speaking with his wife, Macbeth is fully invested in the possibility of killing Duncan. Here, a positive feature of his character complicates the mix: his loyalty to his king. He delivers a soliloquy that praises Duncan and acknowledges how well others think of him. At the end, however, he admits both his own weakness, which he suspects will hold him back from actually doing the assassination, and his "vaulting ambition," which is more likely to win out.

I have no spur

To prick the sides of my intent, but only

Vaulting ambition, which o'erleaps itself

And falls on the other.

Macbeth is fairly credulous. When compared to Banquo, we might even call him gullible or naive. After the Weird Sisters deliver the men's prophecies, Banquo is quick to doubt while Macbeth is ready to believe and does not question the witches' motives or knowledge. After he learns that he has been awarded the title of Thane of Cawdor, he asks his friend, Banquo,

Do you not hope your children
shall be kings,
When those that gave the Thane of Cawdor to me
Promised no less to them? (1.3.128-131)

Macbeth does not stop to consider how the witches might have come to possess this information or whether they might have some evil intent in sharing it with him. Banquo replies to Macbeth, saying,

But 'tis strange,
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray 's
In deepest consequence. (1.3.134-138)

Thus, we see how careful and cautious Banquo is. He does not simply swallow the information the way Macbeth does. Banquo questions and considers and tries to figure out why the witches might have spoken to them. He questions their motives in a way that his more credulous and gullible friend never does.

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.