Macbeth is considered brave. When we first hear about him, he is being praised for his efforts in defeating the Thane of Cawdor. As the bloody sergeant says:
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valor's minion carved out his passage
Till he faced the slave (enotes etext pdf p. 9)
Macbeth was either brave, or out for revenge. His actions were definitely interpreted as bravery.
Duncan tells Malcolm in Act 1, Scene 4, that he totally trusts Macbeth.
There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust. (p. 17)
It is also clear in Act I that Duncan trusts Macbeth. He even seems to feel bad about not promoting him sooner. In Act I, Scene 3:
Would thou hadst less deserved,
That the proportion both of thanks and payment
Might have been mine! Only I have left to say,
More is thy due than more than all can pay. (p. 17)
Duncan completely trusts Macbeth. It is his downfall. He was not at all suspicious about visiting Macbeth’s castle.
I think that trust of the witches also plays a part here. Macbeth clearly believed what they say. When part of the prophecy comes true and he is named Thane of Cawdor, he expects the other part, being king, to come true as well.
If chance will have me king, why, chance
may crown me
Without my stir. (p. 16)
Macbeth thinks he will be king without having to do anything. When he finds out he is not named Duncan’s successor, he gets murderous intentions.
Macbeth seems to be a loyal subject at first. However, the witches twist his thoughts. His loyalty becomes a lie.
The service and the loyalty I owe,
In doing it, pays itself. Your highness’ part
Is to receive our duties, and our duties
Are to your throne and state, children and servants,
Which do but what they should, by doing every thing
Safe toward your love and honor. (p. 17)
Macbeth seems unambitious in the beginning, but note that he lies to Duncan when he names Malcolm his successor. To himself, he shows his rage:
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 (p. 18)
When his wife hears the news, things get even worse as she goads him to murder Duncan.
In Act 1, Scene 5:
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal. (p. 18)
Ambition is definitely something Lady Macbeth possesses in spades.