In interpreting Macbeth's murder of Duncan, there have been psychoanalytic interpretations that include emasculation, incestuous, or even Oedipal fears. Certainly, the spirits that seem to make Macbeth potent, actually make him impotent, according to critic Copp Eli Kahn. This paradoxical motif runs the entirety of "Macbeth," and is evident in Macbeth's defeat of Macdonwald and his murder of Duncan as perverting the natural order of inheritance.
In his murder of Macdonwald in Act I, for instance, Macbeth is described by the captain as having "carved out his passage" (I,ii,19), with his "brandished steel,/Which smoked with bloody execution" as he "carved out his passage" (I,ii,17-19). A suggestion of ending generational continuity exists in these lines. Again, then, in Act II, as Macbeth ponders his murderous deed of the "father" of the country, he sees before him the dagger, which is often interpreted as phallic. Led by this phallic dagger, he approaches Duncan's bed chamber "with Tarquin's ravishing strides (II,i,55). Convinced by Lady Macbeth to become the "serpent," striking up through the "innocent flower"(I,v,64-65). This idea of Oedipal patricide is underscored by Lady Macbeth's being troubled by Duncan's resemblance to her own father. When she warns Macbeth to "consider it not so deeply" ( ), she, in fact, echoes Jocasta's words to Oedipus in Oedipus Rex. She also assumes a murderous maternal role if Macbeth refuses the task of killing Duncan:
Be so much more the man....I would, while [the babe] whie it was smiling in my face Have plucked my nipple from his boneless gums,/And dashed the brains out, had I so sworn as you /Have done to this. (I,vii,51-58)
With her brutal words, Lady Macbeth propels her husband to his ambitious deed; however, Macbeth gives pause before enacting it as, with his vision of the dagger, he ponders his act of "ripping the hereditary body politic untimely from its haven in Duncan's body"*and "knowing what 'twere kill a father" (III,vi,20)---"fair is foul/foul is fair."
Of course, Macbeth and Lady Macbeth's plan to have male children does not come to fruition. Instead, Macbeth's attempt to conceive a new self also becomes skewered as he is left "with a barren sceptre" (III,i,61) and his abuse of the power of his "dagger" leads to his ruin as he is haunted by the foulness of his unnatural deed--"To know my deed, 'twere best not know myself" (II,iii,72)--realizing that "vaulting ambition" (I,vii,27) has destroyed him.
*Robert N. Watson, "Psychoanalytic Interpretations" (essay, 1984)
Before Macbeth kills Duncan he is very nervous, you might even say that he feels dread about what he is about to do. After he has killed Duncan, he is wracked with guilt about what he has just done.
In Act II, Scene 1, Macbeth is readying himself to kill Duncan. His state of mind is best shown by the vision he sees. As he plans, he sees a bloody dagger floating in the air. This shows how worried he is about killing Duncan.
After he kills Duncan he feels even worse. He thinks he heard the servants wake up and a voice condemn him as a murderer. He was so wracked with guilt he could not pray.
Critics have always held Lady Macbeth responsible for Macbeth's deed. She is the one who exploits Macbeth's vaulting ambition to achieve her end. Macbeth never wanted to muder Duncan. This is clearly reflected by his nervous state of mind before the murder. He was perplexed and was wavering between his ambition and his allegiance towards Duncan.
The image of floating dagger that he imagines indicates that he was very disturbed before murdering Duncan. He was experincing some guilt pangs in some remote corner of his heart. Even after committing the murder he has no sense of accomplishment. Infact, his sense of guilt becomes aggravated as he starts imagining and experiencing the false events. This is a reflection of his guilt conscience which finds expression through his disturbed mental and pychological imaginations.