It is important to consider the way that Shakespeare slowly introduces Macbeth as a character to us at the beginning of this play in order to create an impression of him before he even walks upon stage. Note that the audience only sees Macbeth for the first time in Act I scene 3, and in Act I scene 1 he is linked with the evil witches through their announcement of their impending meeting with him and in Act I scene 2 he is described as committing rather excessive acts of violence in battle. Note how the Captain describes what Macbeth did to Macdonald, who rebelled against Duncan:
Like valour's minion,
Carved out his passage till he faced the slave,
Which ne'er shook hands nor bade farewell to him
Till he unseamed him from the nave to th'chops,
And fixed his head upon our battlements.
Even though this act is praised by Duncan, it speaks of a savagery in Macbeth's character that could be suggestive of a profound violence and capacity for evil. Already therefore Shakespeare seems to be setting Macbeth up as a character who may be noble, but whose nobility is definitely tainted and who has the capacity for evil. This impression is further cemented in Act I scene 3, when Macbeth apparently starts at the prophecy of the witches, indicating that he has already thought of killing Duncan and seizing the crown.