Before killing Duncan, Macbeth experiences conflicting feelings about assassinating the king. Although Macbeth desires to become King of Scotland and entertains the idea of murdering Duncan, he recognizes that Duncan does not deserve to die and understands that there will be consequences attached to his actions. Macbeth fears retribution and expresses his concerns in act 1, scene 7 by saying,
If the assassination
Could trammel up the consequence, and catch
With his surcease success; that but this blow
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We’d jump the life to come. But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague th’ inventor (Shakespeare, 1.7.2–10).
After Macbeth lists several logical reasons as to why he should not murder Duncan, he admits that his "Vaulting ambition" is his only driving force and decides to put an end to the plot. However, Lady Macbeth insults his manhood and convinces him to commit the violent crime.
In act 2, scene 2, Macbeth exits Duncan's chamber holding the bloody daggers and is visibly shaken. Macbeth reveals that he is mentally disturbed when he tells Lady Macbeth that he heard Duncan's servants say, "Sleep no more! Macbeth does murder sleep." He also struggles to compose himself and regrets committing the awful crime. Macbeth even refuses to re-enter Duncan's chamber, and Lady Macbeth is forced to replace the daggers.
Macbeth's state of mind is clearly disrupted following King Duncan's assassination. He is filled with remorse and regrets taking the king's life. As he looks down at his hands, Macbeth says,
Whence is that knocking?
How is ’t with me when every noise appals me?
What hands are here? Ha! They pluck out mine eyes.
Will all great Neptune’s ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine,
Making the green one red (Shakespeare, 2.2.71–76).
After Macbeth murders King Duncan, he becomes emotionally unstable and begins to act paranoid and irrational. The guilt, remorse, and anxiety that Macbeth experiences after assassinating the king is the catalyst that leads to his demise.