Quote 1
IAGO
One Michael Cassio, a Florentine,
A fellow almost damned in a fair wife,
That never set a squadron in the field,
Nor the division of a battle knows
More than a spinster—unless the bookish theoric,
Wherein the togèd consuls can propose
As masterly as he: mere prattle, without practice
Is all his soldiership. But he, sir, had the election;
And I, of whom his eyes had seen the proof
At Rhodes, at Cyprus and on other grounds
Christian and heathen, must be beleed and
calmed
By debitor and creditor. This counter-caster,
He, in good time, must his lieutenant be (1.1.21-34)
Here, Iago claims he hates Othello because Othello passed him, Iago, over for a promotion, giving "one Michael Cassio" the job as his military lieutenant instead. Iago claims he's far more qualified than Cassio, who lacks Iago's experience on the field of battle. Clearly, Iago seems pretty jealous. But is this the real reason Iago sets out to destroy Othello? Or, is this merely an excuse to go after him? In other words, does Iago say all of this in order to manipulate Roderigo? (Roderigo, as we soon learn, is completely envious of Othello for marrying Desdemona.)
Quote 2
IAGO
I hate the Moor,
And it is thought abroad, that 'twixt my sheets
'Has done my office. I know not if 't be true,
But I, for mere suspicion in that kind,
Will do as if for surety. (1.3.329-433)
Now this is interesting. Earlier, Iago said he hates Othello because "the Moor" passed him over for a promotion. Yet, here, Iago says he hates Othello because he's heard a rumor that Othello has been hooking up with Iago's wife, Emilia, "twixt [Iago's] sheets." Iago says he doesn't exactly know if the rumor's true, but he's decided to go ahead and ruin Othello's life anyway. Seems like Iago has listed a couple of incompatible motives for seeking to destroy Othello, wouldn't you say? So, we're just not sure we can believe that Iago's jealous of Othello's supposed relationship with Emilia.
Quote 3
Cassio's a proper man. Let me see now:
To get his place and to plume up my will
In double knavery—How, how? Let's see.
After some time, to abuse Othello's ear
That he is too familiar with his wife.
He hath a person and a smooth dispose
To be suspected, framed to make women false.
The Moor is of a free and open nature,
That thinks men honest that but seem to be so,
And will as tenderly be led by th' nose
As asses are.
I have 't. It is engendered. Hell and night
Must bring this monstrous birth to the world's light. (1.3.435-447)
A few lines earlier (see above passage), Iago claimed that he suspects Othello has been sleeping with his wife, Emilia. Here, Iago shares his plot to destroy Othello with the audience – since Othello is so gullible, Iago will lead him "by the nose," making Othello believe that his, Othello's, wife is having an affair with Cassio. Iago plans to plant the seeds of jealousy in Othello. What's interesting about this passage is the way Iago sees his evil plan as a "monstrous birth," a thing that he will bring to "light." What's up with that?