How does Macbeth behave as king in Act III, Scene 1?
In Act III, Scene 1, Macbeth reveals both his duplicity and his unease at wearing the crown. The scene begins with Macbeth and his wife expressing his desire to discuss matters of state (particularly the threat posed by Malcolm, who he blames for the murder of Duncan) with Banquo. When Banquo says he plans to go riding for the day, Macbeth asks him how long and how far he will ride, but not out of curiosity. We learn that Macbeth plans to send murderers after Banquo while he is riding. His reason is that he perceives Banquo, who had also witnessed, and been implicated in, the witches' prophecy in Act I, as a threat, because his children are to be kings, according to the witches:
Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’Tis much he dares,
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valor
To act in safety. There is none but he
Whose being I do fear...
Macbeth fears that if the witches' prophecy was true, as it has been so far, then he, or more accurately, his heirs, would be the real beneficiary of his actions, and he is determined not to let that happen:
He chid the sisters,
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings... If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered...
To make them kings, the seed of Banquo kings!
Rather than so, come, Fate, into the list
And champion me to the utterance!
At the end of the scene, Macbeth summons the murderers, and persuades them that Banquo has acted to keep them from being promoted in the past, and that he is their enemy. He challenges them, in a passage reminiscent of his scene with his wife before murdering Duncan, to murder Banquo and Fleance. Macbeth's duplicity and coldness in plotting the murder of Banquo thus conflicts with his vacillation before murdering Duncan. We see how gaining the crown through murder has already made him a wicked schemer, and we see how recognition of this fact tears at his psyche when he sees Banquo's apparition at the banquet two scenes later.
However, the scene begins with Banquo's declaration that he does not trust Macbeth, who he fears has "play'd most foully for" the crown. Banquo is mindful, however, of the prophecy, which he realizes could hold promise for him as well as Macbeth, and seems to be planning to keep his suspicions to himself. The audience can see that Banquo may, indeed, be an enemy to Macbeth, and that his murder of Duncan has, perhaps, necessitated Banquo's death as well.