Shakespeare uses Iago to show that manipulation is dangerous in Othello. Iago is able to trick almost every character in the play, and this shows the audience that people who seem honest cannot always be trusted.
Firstly, Iago manipulates Roderigo. He takes his money and promises that he will help him win Desdemona, but he never actually does. Iago says "I am not what I am", which shows that he is being dishonest from the very start of the play. The audience knows he is lying but the characters do not, which creates dramatic irony.
Secondly, Iago manipulates Cassio. He gets Cassio drunk even though Cassio says he has "very poor and unhappy brains for drinking". After Cassio loses his job he is very upset about his reputation, saying "Reputation, reputation, reputation!" Iago then tells him to ask Desdemona for help, which is part of his plan to make Othello suspicious.
Finally, Iago manipulates Othello, which is the most important example. He hints that Desdemona is having an affair without ever saying it directly, and he warns Othello to "beware, my lord, of jealousy". Othello then demands "the ocular proof" and Iago uses the handkerchief to trick him. This leads to Othello killing Desdemona at the end of the play.
In conclusion, Shakespeare shows through Iago that manipulation can destroy people. The play warns the audience to be careful about who they trust, because appearances can be deceiving and a person who seems honest may not be.
In Othello, trust is a really big theme and it is shown through lots of different characters. Shakespeare wants us to know that trust is important and that you should be careful about it.
Othello trusts Iago heaps, and this is his biggest mistake. Everyone calls him "honest Iago" and no one works out what he is really like until it is too late. Othello should have trusted his wife instead but he does not, and this causes the tragedy at the end.
Desdemona is really innocent and she trusts everyone. She helps Cassio because she is a nice person and she does not realise that this is making things worse for her. She stays loyal to Othello even at the end, which is quite sad.
Emilia is married to Iago and she gives him the handkerchief. She does not know what he wants it for. At the end she works out what happened and tells everyone the truth, and then Iago kills her for it.
Overall, Othello shows that trust is a big theme. Shakespeare is saying that we should trust the right people, and if you trust the wrong person then bad things can happen to you and to the people you love.
Othello locates its catastrophe not in a lie but in a reputation: Iago spends the play banking the currency of being "honest" precisely so that he can spend it in a single scene, and Shakespeare's comment is that a society which reads trustworthiness off social performance has already built the machinery of its own destruction.
The play establishes early that credit is earned by manner rather than by truth. Iago's opening confession to the audience, "I am not what I am", grants us the one honest relationship in the drama, and the dramatic irony it creates is not decorative: every subsequent use of "honest Iago" by Othello, Cassio and Desdemona alike measures how completely the Venetian habit of judging character by bearing has been exploited. That the word is applied by the manipulated to the manipulator suggests a culture with no instrument for testing sincerity beyond appearance.
Shakespeare then shows that the suspicion Iago exploits was culturally available before he touched it. Brabantio's parting warning that Desdemona "has deceived her father, and may thee" hands the accusation to Othello inside the Senate itself, and Iago merely returns it, almost verbatim, in Act 3. The repetition is the argument: the assumption that a daughter who chooses her own husband is a proven deceiver belongs to Venice rather than to Iago, so his real talent lies in recognising a belief already lying to hand rather than in inventing one.
The mechanism of the temptation scene confirms that the play indicts a system of judgment rather than a single villain. Iago withholds rather than asserts, echoing Othello's own words back as questions and refusing to speak until pressed, so that the accusation arrives as something Othello appears to have deduced himself. His demand for "the ocular proof" is therefore the moment of greatest vulnerability rather than of greatest scepticism, because proof, in a world where a handkerchief becomes evidence by assertion alone, is exactly the thing that can be counterfeited.
The resolution turns, tellingly, on speech rather than sight. The truth surfaces only when Emilia, who has spent the play performing wifely credit, declares that she "will not charm my tongue" and testifies against her own husband, and she is killed within minutes for it. Shakespeare gives the recovery of truth not to evidence nor to authority but to a woman abandoning the very obedience that had made her useful to Iago, which suggests that a society organised around performed trust can be corrected only by someone willing to forfeit her standing entirely. Iago's closing refusal, "Demand me nothing", withholds even the comfort of a motive, leaving the audience with the structure rather than the man: not one liar, but a Venice that could never have caught him.