In Shakespeare's play Macbeth, it is obvious that Macbeth has not involved his wife in planning Banquo's murder. Why did he choose not to involve her?

In Shakespeare’s play Macbeth, Lady
Macbeth is very much involved in the planning of Duncan’s death; indeed, she is the
prime mover in the plot.  However, later, when Macbeth decides to kill Banquo, Lady
Macbeth takes no active role, and indeed Macbeth doesn’t even tell her exactly what he
has in mind (3.1.44-46). Several reasons may help to explain why Macbeth does not
involve his wife in the plotting of Banquo’s death. Among those reasons are the
following:

• By making Macbeth the prime mover in
  the second death, Shakespeare may have wanted to show how much Macbeth’s moral character
  has degenerated between Act 1 and Act 3.


• On the other
  hand, by making Macbeth the prime mover in the second killing, Shakespeare may also have
  wanted to make Lady Macbeth seem less responsible for the second crime, and thus not
  darken her moral character even further. If she had participated in the planning of the
  second murder, she might have seemed an almost Satanic figure; instead, Shakespeare’s
  presentation of her is far more complex.


• In the scene
  immediately after Macbeth dispatches two murderers to kill Banquo, Lady Macbeth appears,
  and it is clear that she is now having second thoughts about the killing of
  Duncan:

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. . . Nought’s had, all’s
spent,

Where our desire is got without
content:

’Tis safer to be that which we
destroy

Than be destruction dwell in dougtful joy.
(3.2.4-7)

Perhaps, then,
Macbeth did not want to run the risk that his wife might have some qualms about another
killing.  If she were to have such qualms, she might conceivably have tried to dissuade
him from the second murder. After all, she later misses a perfect opportunity to suggest
that he kill Banquo (3.2.37-38).

• Perhaps, too,
  Macbeth did not want to trouble his wife any further than she was already
  troubled.


• Perhaps Macbeth, having been shamed by his wife
  into killing Duncan, wants to prove to her now that he can act on his own. This
  possibility is suggested when he says to her, concerning Banquo’s
  murder,

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Be innocent of the knowledge, dearest
chuck,

Till thou applaud the deed.
(3.2.45-46)

Such phrasing
implies that Macbeth expects his wife to approve of the killing of Banquo and that he
even takes a kind of pride in having concocted the plan on his
own.