One way to deal with the topic of good and evil as it is presented in William Shakespeare’s play Othello might be to rank the characters along a spectrum ranging from severe evil to exemplary goodness. Iago, obviously, might represent one end of the spectrum; Desdemona might represent the other. One might then discuss where and why each of the other characters falls in this spectrum. Which characters are closer to Iago on the spectrum, and which characters are closer to Desdemona? One might also wish to discuss how even Iago and Desdemona may not be “pure,” “absolute” exemplars of good vs. evil. Even Desdemona, for instance, lies to Othello about the handkerchief, and even Iago feels that he has been wronged and injured by others. Nevertheless, Iago is definitely at one far end of the spectrum, and Desdemona seems just as definitely at the other far end.
One might then discuss the other characters, perhaps moving from relatively minor characters (such as Brabantio) to definitely major ones (particularly Othello himself). Each of these characters could be compared and contrasted with Iago on the one hand and Desdemona on the other. Such comparison and contrast would allow an analysis to be detailed and specific: one would not be evaluating the characters in terms of large, vague, abstract ideas of good vs. evil but in terms of the actual examples of good and evil exemplified by Desdemona and Iago, respectively. Using this method, one would also be implicitly comparing and contrasting all the characters with one another, not simply with Iago and Desdemona.
In short, this approach might result in an analysis that was lengthy, comprehensive, detailed, specific, and focused.
In trying to show how Iago and Desdemona exemplify different ends of the moral spectrum, one might focus on particular speeches given by these two characters that seem to reveal their basic moral natures. Practically any of Iago’s early exchanges with Roderigo might provide good evidence, as when he tells Roderigo, concerning Othello,
I follow him to serve my turn upon him. (1.1.42)
Or one might look at Iago’s final words in Act 1. For Desdemona, one might look at her first speeches, in which she defends her marriage to Othello, as when she says,
I saw Othello’s visage in his mind,
And to his honors and his valiant parts
Did I my soul and fortunes consecrate. (1.3.253-55)
It might be best to find early speeches by Iago and Desdemona with which to sketch their essential moral characters, so that speeches and characters who come later can more justly be compared and contrasted with the speeches by these two moral opposites.