Macbeth has succeeded in his original goal of becoming King of Scotland. But he is not happy. He is afraid of Banquo, not only because the Witches have prophesized that Banquo's heirs will be kings, but because he is aware that Banquo understands how he came to be king by murdering Duncan and forcing Duncan's sons to flee for their lives. In the previous scene Macbeth talks about the fears he is experiencing now that he has become king. "Our fears in Banquo / Stick deep, and in his royalty of nature / Reigns that which would be feared. '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." (3.1.53-59) Macbeth has never known fear before, but now it is a constant companion because it goes with his guilt. (Guilt is largely fear of punishment.) In Act 3, Scene 2, he expresses envy of Duncan, because the dead king no longer has all the concerns that go with the position. The following excerpt contains some of Shakespeare's most beautiful writing:
"Duncan is in his grave, / After life's fitful fever he sleeps well. / Treason has done his worst; nor steel nor poison, / Malice domestic, foreign levy, nothing / Can touch him further." (3.2.25-29) Note the alliteration in "life's fitful fever."
Macbeth is becoming a nervous wreck because he is unable to sleep. This was foretold in Act 2, Scene 2, right after he had murdered the King. "Methought I heard a voice cry 'Sleep no more! / Macbeth does murder sleep' -- the innocent sleep, / Sleep that knits up the raveled sleave of care, / The death of each day's life, sore labor's bath, / Balm of hurt minds, great nature's second course, / Chief nourisher in life's feast." (2.2.47-52)