In Shakespeare's Macbeth, how does the killing of Banquo compare and/or contrast with the murder of Duncan? What change in Macbeth's character do...

The planned killings of Duncan and Banquo in Shakespeare’s
Macbeth differ in significant ways. These differences emphasize
various changes in Macbeth’s character. Particularly different are the ways Macbeth
contemplates the killings.  Among the differences are the
following:

• When contemplating the killing of
  Duncan, Macbeth is full of hesitation, ambivalence, and doubt, as when he says that the
  king is present at his house

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. . . in double
trust:

First, as I am his kinsman and his
subject,

Strong both against the deed [that is, the
murder]; then, as his host

Who should against his murderer
shut the door,

Not bear the knife myself . 
(1.7.12-16)

However, when
plotting Banquo’s killing, Macbeth shows very little hesitation at all
(3.1.48-140).

• When contemplating the murder of
  Duncan, Macbeth has to be urged on by his wife (1.7.31-82), but when contemplating the
  murder of Banquo, Macbeth acts without her prompting
  (3.1.48-140).


• When Lady Macbeth tries to persuade Macbeth
  to go through with the killing of Duncan, she implies that he is not truly a man if he
  will not commit the deed (1.7.47-49). When Macbeth urges the “two murderers” to
  assassinate Banquo, he uses much the same
  argument:

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Now if you have a station in the
file

Not i’ the worst rank of manhood, say’t . . . .
(3.1.102-04)

Macbeth, in
other words, has come very much to resemble his wife, at least as she behaved in Act 1,
scene 7. He is now cunning, ruthless, manipulative, and determined, and he is also
effective in the way he plays on the feelings of others. Thus he urges the murderers to
kill Banquo by telling them that Banquo’s death is in their own
self-interests:

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. . . I will put that business in your
bosoms

Whose execution takes your enemy
off,

Grapples you to the heart and love of us . . .
(3.1.104-06)

Lady Macbeth had
used similar arguments when trying to persuade Macbeth to kill Duncan
(1.7.41-45).

• Even after he had earlier decided
  to kill Duncan, Macbeth had still referred to the murder as “this terrible feat”
  (1.7.80). Later, however, when urging the murderers to kill Banquo, he shows no such
  qualms (3.1.104-07).


• In the discussions preceding the
  death of Duncan, Lady Macbeth was the one who formulated concrete, specific plans
  (1.7.61-72); in the discussions preceding the killing of Banquo, it is Macbeth who
  formulates the specific plans (3.1.128-39).

In
short, before the killing of Banquo, Macbeth’s moral character has greatly deteriorated.
He is not the man of troubled conscience he had been before the murder of Duncan.
Instead, he is now more obviously evil and vicious.