How does Shakespeare make Macbeth a character with whom the audience can sympathize?

Shakespeare is notoriously ambiguous in his tragedies,
which contributes to their popularity among teachers and scholars, since they can be
debated relentlessly.  The last I heard, for instance, more commentary had been written
about Hamlet than any other book in existence, except the
Bible. 

Macbeth
is ambiguous, as well.  For instance, many productions of the play show
Banquo's ghost in Act 4.3 at the feast.  The ghost's appearance seems likely to be real,
or actual, in a play filled with the supernatural.  Shakespeare uses a ghost in
Hamlet, why not in Macbeth?  Also, Gertrude in
Hamlet doesn't see the Ghost of King Hamlet during the bedroom
scene when the Ghost is seen by Hamlet, so there is nothing unusual about no one else
seeing Banquo's ghost except Macbeth. 

And if the ghost is
real, then Macbeth is not insane.  I think we misinterpret when we place too much
emphasis on Macbeth's so-called insanity, and relieve him of responsibility by doing
so.  If Macbeth is not responsible, it is because of an overall design by fate or
predestination, an issue important in Elizabethan England due to the Protestant
Reformation, which brought the issue to the attention of
Europeans. 

That said, any sympathy for the character of
Macbeth the audience feels must come from some other source than his insanity.  And it
must not come from feeling that he isn't responsible--he is.  The witches tell him only
that he will be king:  he turns that into the thought that he needs to be king now!  He
turns the prediction into the need to kill Duncan.  Also, he seeks the witches out in
Act 4.1, not the other way around.  He also makes the mistake of cutting his wife out of
the decision-making process once Duncan is dead:  he kills the grooms, orders the
murders of Banquo and Fleance and Macduff's family, all on his own.  These are all
mistakes he makes, without anyone else's help.

So where
does sympathy come from?  His nobility in defending Duncan at the beginning of the play;
his nobility in death at the close of the play; his hopeless situation once he is
surrounded--he is like a bear chained to a tree, attacked by a pack of dogs for the
entertainment of an audience (Act 5.7); from the intellectual side of his personality
that recognizes everything he's done is meaningless (the "Tomorrow" speech, Act
5.5).

Sympathy for Macbeth is not in any way central to the
play.  His ambition and fall from grace are central.  But if one does feel sympathy for
him, it comes from the above.  He allows himself be deceived by the witches and talked
into Duncan's killing by his wife (and these are conscious decisions).  And he takes
over from there and brings about his own downfall.  That cannot be dismissed by any
insanity.  He is guilty.  But sympathy can exist for him, anyway.  Not because we take
the guilt away, but because we see other characteristics than just evil. 
 

In Shakespeare, in sophisticated literature, characters
are mixtures of positive and negative character traits, as are actual human beings.  We
don't need to make excuses for Macbeth in order to feel sympathy for
him.