In Macbeth, who is to blame for the death of Duncan?

This question, actually is one that is very much debated
in regards to this play.  In order to answer it satisfactorily, you must define "to
blame."  Do you mean who actually wielded the knife?  No question, then, it's Macbeth
himself.  Though we don't see the murder onstage, he and Lady Macbeth have a scene --
Act II, scene ii -- in which Macbeth has just come from the
murder.

However, "blame" is often distributed, in a murder,
to those who function as accessories to the crime.  In this case, is Lady Macbeth also
"to blame?"  How about The Witches?  Let's look at both of these possibilities
individually.

First, Lady Macbeth, in the scenes before the
murder, seems to drive the action forward.  In Act I, scene v, she all but declares the
murder a done deal.  She says that "never/[s]hall sun that morrow see," referring to
Duncan's staying the night at their home, but not living to see the next day.  She ends
this discussion with a reluctant Macbeth by saying, "Leave all the rest to me," implying
that she will see that the murder is committed.  And the actual plan, divulged in Act I,
scene vii, is given to Macbeth by Lady Macbeth.  She
says:

. .
.When Duncan is asleep

. . .his two
chamberlains

Will I with wine and wassail so
convince

That memory. .
.

Shall be a fume. . .when in swinish
sleep

Their drenched natures lie as in a
death,

What cannot you and I perform
upon

The unguarded
Duncan?

This plan implicates
Lady Macbeth as being the mastermind behind Duncan's death, and, as such, partly to
blame for the murder.

The Witches are also considered by
some to share in the blame, since their prediction of Macbeth's becoming King is what
leads him first to consider the idea.  Have they poisoned his brain in some way, created
some evil spell that has caused him to act against his better nature in killing Duncan? 
Well, this can only be a supposition, there is not evidence in the text to prove it. 
But it is certainly worth considering that the Witches, through their supernatural
intervention, also share some of the blame for Duncan's
death.

So, Macbeth is the actual murderer in the play, but
Lady Macbeth and the Witches could both be considered to share in the blame for the
death of Duncan.

The links below will give more detail for
considering this question further.