Iago is a very manipulative character who does everything he can to destroy Othello. From the start, he uses Roderigo as a puppet to get at Othello by promising he will get what he wants. Since Roderigo wants Desdemona, he is easily manipulated into going to Brabantio’s house in the middle of the night to tell the father of his daughter’s elopement. Once there, Iago vulgarly shouts for all to hear:

I am one, sir, that comes to tell you your daughter

And the Moor are now making the beast with two backs.

His lewd comment makes Brabantio immediately search for his daughter, intent on destroying the man who has stolen her. Iago bows out of the controversy, telling Roderigo he cannot be seen there since he is supposed to appear loyal to Othello. He assures him, however, that he has no loyalty whatsoever and only pretends for his own gain.

Roderigo continues to believe Iago’s insistence that Desdemona will leave Othello for him. He even gives Iago jewelry supposedly to woo Desdemona, never realizing until it’s too late how the evil Iago is using him. Roderigo believes the lies that he should “put money in thy purse” and Desdemona will be his soon enough.

The reader is privy to Iago’s innermost thoughts, as he confesses in his soliloquys his true intentions “to abuse Othello’s ear” because Othello “will as tenderly be led by th’ nose/ As asses are.” Iago has no intention of letting Othello be happy, and he is willing to use innocent people to retaliate against a perceived wrong. He has no qualms about “enmesh[ing] them all.”

Iago manipulates Othello the most, immediately pretending to have defended him during the nighttime drama at Brabantio’s. However, his evil scheming continues to worsen as he sets out to convince Othello of his wife’s infidelity. His assertion that all women cheat and insinuation that Desdemona is too friendly with Cassio are only the beginning:

In Venice they do let God see the pranks

They dare not show their husbands. Their best conscience

Is not to leave ‘t undone, but keep ‘t unknown.

Iago’s reminder that Desdemona lied to her father in order to marry Othello compels Othello to wonder about whether his wife is like the Venetian women that Iago speaks of.

Continuously making up lies, Iago thoroughly disgusts Othello by preying on his insecurities:

I lay with Cassio lately …

In sleep I heard him say “Sweet Desdemona,

Let us be wary, let us hide our loves.”

And then, sir, would he gripe and wring my hand,

Cry “O sweet creature! then kiss me hard,”

As if he plucked up kisses by the roots

That grew upon my lips; then laid his leg

O’er my thigh, and sighed and kissed, and then

Cried “Cursed fate that gave thee to the Moor!”

Iago’s evil lie implants a vision in Othello’s head is too much for him to bear.

But Iago is not finished yet. Using the handkerchief and a conversation with Cassio that is easily misconstrued, Iago pushes Othello to the limit:

And to see how he prizes

The foolish woman your wife! She gave it him, and

He hath giv’n it his whore.

Not satisfied with killing innocent people, Iago urges Othello to strangle Desdemona in bed, “the bed she hath contaminated.” The demonic Iago does not care at all that he is responsible for destroying so many lives; all that concerns him is revenge.

Firstly, the fact that Iago declares his intention to harm Othello when he speaks to Roderigo, is a good quote:

I follow him to serve my turn upon him

Iago clearly and unambiguously says here that he only shows obedience to Othello to fool him into believing that he is being loyal and servile so that he may plot his downfall - true to the expression 'Keep your enemies closer.'

In the same speech he tells Roderigo:

... Others there are
Who, trimm'd in forms and visages of duty,
Keep yet their hearts attending on themselves,
And, throwing but shows of service on their lords,
Do well thrive by them and when they have lined
their coats
Do themselves homage: these fellows have some soul;
And such a one do I profess myself. 

It is obvious in these lines that Iago has much admiration for the type of servant who only obeys his master out of show and not because of genuine respect or duty. His sole purpose is to serve his own needs - they do homage to themselves. Such persons show spirit and Iago perceives himself to be the same. A confession of his devious and pernicious nature.

Iago has planned to rouse Brabantio, the beautiful and chaste Desdemona's father, to inform him that Othello had kidnapped her. The plan is to upset Brabantio to such an extent that he would take action against Othello. When the two arrive at his house, Iago shouts that there were thieves around. When Brabantio wakes and enquires what all the noise is about, Iago declares:

Zounds, sir, you're robb'd; for shame, put on
your gown;
Your heart is burst, you have lost half your soul;
Even now, now, very now, an old black ram
Is topping your white ewe. Arise, arise;
Awake the snorting citizens with the bell,
Or else the devil will make a grandsire of you:
Arise, I say.

Iago uses the most disgusting animal metaphors to inform Brabantio that his daughter is being sexually abused. He cleverly refrains from mentioning the so-called abuser by name, but makes indirect references to Othello. He is incessant and continues using these images to further shock Brabantio, who later seeks out the duke and demands Othello's arrest.

Iago later maliciously informs Othello about how badly Brabantio had spoken about him:

Nay, but he prated,
And spoke such scurvy and provoking terms
Against your honour
That, with the little godliness I have,
I did full hard forbear him.

He tells Othello that Brabantio criticized him at length and used such shocking and provocative references that he, Iago found it difficult to restrain himself from lashing out at Brabantio. Iago has now manipulated both men and ruined, in a very short time, the good relationship (though superficial) that existed between the two.

Iago also masterfully manipulates the foolish Roderigo to do his bidding by dangling the fact that he would help him pursue Desdemona's affections and win her over in front of him like a carrot. He tells Roderigo:

Thou art sure of me:--go, make money:--I have told
thee often, and I re-tell thee again and again, I
hate the Moor: my cause is hearted; thine hath no
less reason. Let us be conjunctive in our revenge
against him: if thou canst cuckold him, thou dost
thyself a pleasure, me a sport.

He tells Roderigo that they should work together against Othello. In this venture, both will achieve what they want - he revenge and Roderigo Desdemona.

There are many other examples of Iago's manipulations but the one which stands out particularly since it finally convinces Othello of Desdemona's 'infidelity', is Cassio's speech with Bianca. Iago had planted Desdemona's handkerchief in Cassio's room and he is now speaking to him. Othello is in hiding, eavesdropping. Cassio speaks:

She was here even now; she haunts me in every place.
I was the other day talking on the sea-bank with
certain Venetians; and thither comes the bauble,
and, by this hand, she falls me thus about my neck--

Othello thinks that Cassio is speaking about his wife, but he is speaking about Bianca. She arrives later and angrily confronts Cassio:

Let the devil and his dam haunt you! What did you
mean by that same handkerchief you gave me even now?
I was a fine fool to take it. I must take out the
work?--A likely piece of work, that you should find
it in your chamber, and not know who left it there!
This is some minx's token, and I must take out the
work? There; give it your hobby-horse: wheresoever
you had it, I'll take out no work on't.

When Othello sees the handkerchief, he is fully convinced that he has been cuckolded. His mind is made up and he sets in motion the tragic events which transpire later.

What quotes in Shakespeare's Othello portray Iago's betrayal of Othello?

Iago is depicted as the ultimate villain, carefully planning and executing an impressive plot to get revenge on Othello. He seeks revenge because Othello did not promote him to the revered position of lieutenant and gave the title to the inexperienced Michael Cassio. Iago also suspects that Othello has slept with his wife and proceeds to manipulate several characters in order to get revenge. Despite Iago's malevolent intentions, Othello completely trusts him and believes that he is a benevolent, loyal subject.

Iago's betrayal is poignantly depicted in act three, scene three, when he initially begins to plant the seeds of jealousy and suspicion into Othello's head. When Othello witnesses Cassio run away after speaking with Desdemona, Iago insinuates that Desdemona and Cassio are engaged in an affair and warns Othello about the dangers of jealousy. Iago proceeds to say,

I speak not yet of proof.
Look to your wife, observe her well with Cassio.
Wear your eyes thus, not jealous nor secure.
I would not have your free and noble nature
Out of self-bounty be abused. Look to ’t.
I know our country disposition well.
In Venice they do let God see the pranks
They dare not show their husbands.
Their best conscience
Is not to leave ’t undone, but keep’t unknown. (3.3.201-209)

Iago continues to betray Othello by painting Desdemona in a negative light and reminding him of how she cleverly deceived her father. Iago's portrayal of Desdemona raises concerns about her honesty, and Othello begins to entertain the idea of his wife's infidelity. Iago continues to play on Othello's insecurities by saying,

Ay, there’s the point. As, to be bold with you,
Not to affect many proposèd matches
Of her own clime, complexion, and degree,
Whereto we see in all things nature tends—
Foh! One may smell in such a will most rank,
Foul disproportions, thoughts unnatural.
But—pardon me—I do not in position
Distinctly speak of her, though I may fear
Her will, recoiling to her better judgment,
May fall to match you with her country forms,
And happily repent. (3.3.234-244)

Once Othello begins to doubt Desdemona's faithfulness, Iago leaves him with his tortured thoughts and receives Desdemona's handkerchief from Emilia. Iago proceeds to elaborate on the rest of his plan during an aside by saying,

I will in Cassio’s lodging lose this napkin
And let him find it. Trifles light as air
Are to the jealous confirmations strong
As proofs of holy writ. This may do something.
The Moor already changes with my poison. (3.3.331-335)

Overall, Iago's betrayal of Othello is depicted in act three, scene three, when he insinuates that Desdemona is having an affair with Cassio and manipulates Othello's insecurities. Othello becomes tainted by Iago's suggestions and becomes suspicious of his wife.

What quotes in Shakespeare's Othello portray Iago's betrayal of Othello?

A particular quotation from act 1, scene 3 is a great example of Iago's treachery:

Thus do I ever make my fool my purse,
For I mine own gained knowledge should profane
If I would time expend with such a snipe
But for my sport and profit.

Iago has just sent the hapless Roderigo off to sell his land. He told the foolish young man that he needed to do this in order to raise enough money to woo Desdemona, with whom Roderigo is head over heels in love. But in actual fact, the wicked, scheming Iago intends to cheat Roderigo out of his money. This is what he means when he says that he has made a fool his own purse.

It's at this point in the play that we realize just how wicked Iago really is. He will stop at nothing to get his own way, even if it means stealing from those ostensibly on the same side as himself. Iago is a user, a master manipulator with the ability to get other people to do his own bidding. And all the while they never know they're being duped until it's too late.

So will I turn her virtue into pitch
And out of her own goodness make the net
That shall enmesh them all.

These lines come from act 2, scene 3. Here, Iago is referring to his dastardly scheme to destroy Desdemona. He's going to trash her reputation, turning her virtue into pitch, which is a kind of black, sticky substance. He then mixes his metaphors by saying that he will make a net out her goodness that will trap everyone else he wants to destroy. Iago cleverly realizes that by destroying Desdemona's reputation for virtue, he'll also destroy his enemies with one fell swoop.

What quotes in Shakespeare's Othello portray Iago's betrayal of Othello?

Iago's betrayal is a central theme in Shakespeare's Othello, and, therefore, the play is full of lines portraying his betrayal. While we are limited to space, below are a few ideas of types of quotes to look for that help prove and demonstrate his betrayal.

One good type of quote to look for is anything relaying the reasons for his betrayal. Iago betrays Othello out of jealousy because Othello chose not to promote him to lieutenant, despite his merit, but instead chose another officer. Therefore, any quote explaining what Iago perceives as Othello's offense will help prove that Iago betrayed Othello by showing us the reasons why. For example, in his first speech, Iago argues that he knows he would be valuable as a lieutenant and should have been chosen:

I know my price, I am worth no worse a place.
But he, as loving his own pride and purposes
Evades them with a bombast circumstance
...
And, in conclusion, ... says he,
"I have already chosen my officer." (I.i.11-17)

The second type of quote would be any quote discussing Iago's plans for betrayal. In Act II, Iago observes Desdemona and Cassio flirting and concocts his plan to make Othello jealous. Not only that, he believes that Othello has slept with his own wife and therefore wants to avenge himself, not just for failing to promoting him to lieutenant but also for sleeping with Iago's wife, as we see him state:

And nothing can or shall content my soul
Till I am even'd with him, wife for wife;
Or failing so,, yet that I put the Moor
At least into a jealousy so strong
That judgement cannot cure. (II.i.306-310)

All of these types of lines will help convey and prove Iago's betrayal of Othello.