How has Shakespeare used Desdemona and Iago to portray the good and evil of human nature?I have to write a presentation on an aspect of Othello and...

You might discuss the difficulty of judging good and evil.
 Othello is a good man--honorable, brave, skilled, but he makes a horrible error in
judgment.  In Act 3, scene 3, Othello chooses Iago over Desdemona.  It is not
necessarily because he is tempted; rather it was because he was deceived.  Iago appears
to be good ("I am not what I am"), but in actuality is evil incarnate.  He turns
Desdemona's "virtue black as pitch" in Othello's eyes, and what appears to be evil to
Othello is actually good.

This mistake has devastating
consequences.  Since Othello is the military head of state in Cyprus, he can exact the
punishment.  Desdemona's supposed infidelity is equated to treason, and Othello feels
that he has no choice but to execute her ("It is the cause").  At the same time that
Othello is contemplating Desdemona's death, he bonds more solidly with Iago.  The end of
Act 3, scene 3, is reminiscent of a wedding scene in which two people exchange vows.  In
this case the vows that are exchanged are  Iago and Othello's commitment to murder the
innocents:  Cassio and Desdemona.

If I were writing this
paper, I would focus on Othello's choice--his belief that he is is right, that he is
just.  You could from there explore how Iago manages to manipulate Othello and why
Othello is vulnerable to Iago's manipulation.  You might also explore the Christ imagery
used to characterize Desdemona--especially in her death scene in Act 5.  She dies asking
for Othello's blessing and attempting to place the blame for her death on
herself.

At the end of the play, Othello realizes that he
has been duped. He calls Iago a "demi-devil that hath ensnared [his] soul and body."
 Othello's mistake and realization are universally relevant.  It is so easy to misjudge
those around us, to become suspicious because of our own failings, to embrace those that
in reality want to do us harm.  Shakespeare's message is as relevant to us as it was to
17th century audiences:  sometimes is is very difficult to distinguish the good from the
bad guys, but it is vitally important that we do so.