How is the theme of witchcraft used in Shakespeare's Othello?

Witchcraft is prominently raised as a major theme in
Shakespeare’s Othello in the first act, when Brabantio suspects
that his daughter Desdemona may have been influenced by witchcraft to fall in love with
Othello and marry him. When Brabantio first encounters Othello, he
exclaims,

O
thou foul thief, where hast thou stowed my daughter?

Damned
as thou art, thou hast enchanted her!
(1.2.61-62)

He then develops
these accusations at length. Later, when both men appear before the ducal court at
Venice, Brabantio again publicly charges Othello with having used “spells and medicines”
to steal Desdemona (1.3.61). He says that she would never have consented to the marriage
if “witchcraft” had not been used (1.3.64).  He elaborates further on such charges when
he accuses Othello of “practices of cunning hell”
(102).

Othello, in defending his conduct and explaining the
mutual love between himself and Desdemona, concludes,

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She loved me for the dangers I had
passed,

And I loved her that she did pity
them.

This only is the witchcraft I have used.
(1.3.166-68)

Shakespeare goes
out of his way, then, to raise in the first act of the play the idea of humans in league
with the devil.  This idea does not fit Othello (at least not until the very end of the
work perhaps, when he is accused by Emilia of devilish conduct), but it certainly seems
appropriate to Iago.  Again and again Iago acts almost as a Satanic figure who takes
pleasure in evil and who for the most part lacks any kind of conscience. At the very end
of the first act, for instance, when he has conceived the plan by which he will try to
destroy Othello, he says,

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I have’t! It is engendered! Hell and
night

Must bring this monstrous birth to the world’s light.
(1.3.392-93)

Part of the
paradox of Iago’s behavior, however, is that he is less a witch (or a man with
supernatural powers) than he is a coldly calculating, supremely rational man who
corrupts God’s great and heavenly gift of reason in order to do evil.  Instead of using
reason to tame his passions, he uses reason to plot and scheme and thus advance his
passionate desires.  In the end, there is very little that is magical about his
manipulation of Othello; just as Othello uses no witchcraft, neither does Iago. 
Instead, Iago plays on Othello’s own passions and corrupts and undermines Othello’s own
powers of reason. Othello, at the end of the play, openly compares Iago to a devil
(5.2.285-86), but, by the conclusion of the work, witchcraft seems no more responsible
for Desdemona’s death than it was for Desdemona’s marriage.