Shakespeare almost uses Banquo's murder as a foil to the original murder of Duncan. By comparing the two, the audience can truly see how embroiled in his own schemes Macbeth has become and also the full degree of his ruthless commitment to achieving his ambition. Macbeth had many doubts about carrying out Duncan's murder and had to be prodded and convinced by Lady Macbeth to carry it out. Even with all of her persuasion, Macbeth is still worried about the moral repercussions of such an act, doubting his role as murderer:
"First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,(15)
Not bear the knife myself" (I.vii. 13-16)
Clearly, Macbeth has reservations about carrying out Duncan's murder. He doubts his ability to execute the deed and is plagued with self-doubt and feelings of guilt and possible recrimination.
Contrastingly, Macbeth's attitude has become much more cut-throat (no pun intended!) the second time around when he plans Banquo and Fleance's murders. Shakespeare uses these scenes to show the audience how much more comfortable Macbeth has become in his role as murderer; he now confidently plans their deaths and hires the hitmen himself, with no prodding or convincing required by Lady Macbeth. He has taken matters into his own hands; Macbeth's detachment from his earlier moral stance becomes evident in his later conversation with the head murderer:
"Thou art the best o’ the cut-throats! Yet he's good
That did the like for Fleance. If thou didst it,
Thou art the nonpareil" (III.iv.18-20).
In Banquo's death, Macbeth is much more at ease-- he practically celebrates the fact that Banquo has been killed and then mourns the fact that Fleance escaped. His response to his former ally's murder reveals how far Macbeth has slipped in terms of morality. His ambition for the throne has blinded him to the moral wrongness of murder.