What is Macbeth's role in the play Macbeth?Including themes and issues where relevant

One of the great things about this play is that it shows
us about governance and power.  To state the theme quite simply, it is, Power corrupts
and absolute power corrupts absolutely.

Consider what
happens.  At the beginning of the play, we meet Macbeth, the protagonist and tragic hero
of the play.  He is a good man it would seem.  A valiant warrior.  A loyal
Scotsman.

Is he ambitious?  Of course he is.  This was not
a time in Scottish history where the son necessarily followed his father onto the
throne.  With constant threats from the raiding Norsemen, a boy king would not have been
a good thing.  You needed a strong and proved military leader on the throne.  Does
Macbeth, at the beginning of the play fit the bill?  Yes.  Does Malcolm?  No. he has not
proved himself in battle.  In fact, he had to be rescued...by
Macbeth.

The prophsey that he will be king in the future
seems thwarted by Duncan naming his son, Malcolm as his successor.  Does Macbeth feel
cheated?  Yes.

Does Lady Macbeth feel that her husband
rather than a seemly weak and untested youth (Malcolm) deserves to be king? 
Yes.

The combination of Macbeth's ambition, the weird
sister's prediction, and the urging of Lady Macbeth all combine to lead Macbeth from
hero to tyrant.

Once he murders Duncan. he must continue to
murder to cover up his actions.  We hear how Scotland has become a horrible place to
live.  Fear and distrust are the norm under Macbeth.  Neighbor spys on neighbor.  People
don't know who they can trust.  He does not use his power wisely or for the good of his
subjects.  He uses his power to control and punish those who don't fall in line.  Look
what happens to the Macduff family.

In the play
The Best Man, the old and dying former president tells one of the
candidates, "Power is not a toy we give to good children.  It is a weapon and the strong
man takes it and he uses it."

Shakespeare shows us this
terrible lesson in Macbeth.  He uses his power to beat his people
over the head.  One of the great lessons of history is the corrupting nature of power. 
Once Macbeth killed Duncan he was corrupted by power.