Who should bear the final responsibilty for the tragedy in Macbeth? I have to write an essay answering this question and I just wanted some...

Concerning your essay on Shakespeare's
Macbeth, first, write your introduction last--you don't need to
worry about that yet.  You need a thesis and supporting points with evidence.  Ideas for
introducing your topic will probably come to mind while you're writing the rest of your
essay.  Prewriting, organizing, and writing the "meat" of the essay usually comes
first--that's what the essay is about.  By the time you've learned your topic through
writing, the introduction will probably come
easily. 

Concerning the body of your essay, you need three
or four pieces of evidence that demonstrate that Macbeth is responsible for the
tragedy.  Numerous examples exist, so this should be no problem.  Some possibilities
follow:

• Macbeth reasons the decision out in Act
  1.7.  At one point, he decides not to go through with the killing of Duncan.  This
  demonstrates that the decision is in Macbeth's hands.  If his wife berates him and
  manipulates him into changing his mind, he allows himself to be convinced and
  manipulated.  Plus, he does the actual killing.   


• The
  decision to kill Banquo and Fleance is entirely Macbeth's own.  Macbeth chooses not only
  to do it, but also chooses the killers and makes the plan.  This casts serious
  suspicions on Macbeth (too many coincidences favoring Macbeth, Act 3.6), and the attempt
  to kill Fleance fails.


• The decision to kill Macduff's
  family, because he can't get to Macduff who is in England, is entirely Macbeth's own. 
  This is almost a temper tantrum--he can't go after the defiant Macduff who skips his
  parties, so he takes his wrath out on Macduff's family instead.  Killing Macduff's
  family serves little or no political or military purpose for Macbeth, but it certainly
  solidifies the opposition against him (Act
  4.3).

For your rebuttal paragraph or two (a
section that rebuts arguments against your thesis) you could point out that the witches
say only that Macbeth will be king--they don't say anything about murdering Duncan. 
And, even if the witches are supernatural and know the future, knowing and causing are
two different things.  The decisions made are still Macbeth's to
make.