A possible motive for Iago is class-resentment, and indeed, this is the only motive he expresses with any consistency.
Racist interpretations of the play sometimes see Desdemona as am empty-head strumpet who only marries Othello because she can't control her libido and condemn her for "miscegenation". See the quotation from John Quincy Adams below. Of course, Adams' frothing about Desdemona's 'betrayal' of duty, race, sex, and country also stems from his ideas about everything women should be striving for — to be the perfect consorts of men like their own families (and betraying one's own sex presumably meaning not keeping women of one's own station an exclusive commodity to men of one's own status or preferably higher, therefore presumably degrading their value), and churning out sons in the image of one's own people. Desdemona's relationship with Othello is in this respect behaving as if she's her own individual person, rather than a conduit for Venetian sons — something unforgivable for misogynists.
There are alternative views of Othello himself: Hero, fool or downright monster? The critic John Sutherland noted that in the original story, Shakespeare's source material, the unsympathetic Othello-equivalent plotted with the Iago-equivalent about how to kill his wife in a way that wouldn't leave a mark/would look like natural causes, so that he could escape punishment and could maintain his position. While Shakespeare's Othello is way more sympathetic overall, there's a few lines that indicate that he too attempted such a plan, putting him in a worse light. Sutherland also discusses how despite making grandiose claims about his handkerchief, in other instances, Othello treats it like a normal handkerchief, and his later obsession with it has elements of Believing Their Own Lies.
At one point, Othello has a seizure according to Iago. If Othello truthfully has epilepsy, then it's one more obstacle that makes him different from the Venitians and might feed into his insecurity.
According to Othello, Desdemona once said that she wished God had made her a man that could go on the adventures Othello's described. The obvious, and indeed, most common interpretation is that Desdemona wishes God had made her a man to marry, since it's established that Othello's strength and bravery are are a huge part of what won her over. However, others have suggested that Desdemona means she wishes God had made her a man, so that she could go on these adventures herself. The former is probably what was meant, but it's interesting to think about, and potentially throws Desdemona's character in a different light. (And, Shakespeare being Shakespeare, the double-meaning may not have been an accident.)
One of Iago's alleged motives is that he heard a rumor that Emilia cheated on him with Othello. There's no indication that this is true, and most readers agree that it's probably not, but Emilia does have a speech where she claims that adultery can be (and often is) justified, particularly if a woman cheats on her abusive husband. Iago is certainly awful to Emilia, and Emilia overall demonstrates a far more cynical and "practical" outlook on marriage (and life in general) than Desdemona. It's not too far-fetched to think that she may have sought companionship elsewhere — if not with Othello, then with somebody.
Draco in Leather Pants: Modern productions tend to be more sympathetic towards Iago, perhaps overly so. The fact that he was once played by Kenneth Branagh—and more recently by Ewan McGregor—doesn't help.
Ensemble Dark Horse: Emilia, Iago's wife and Desdemona's maid, has been extremely popular throughout much of the play's history for the way she excoriates unfaithful husbands in a touching monologue in the fourth act, chews out Othello for his crime, and delivers the fatal blow to her own husband's Evil Plan, gets stabbed, and then keeps going. She's even triumphant about being stabbed, as the act shows Iago up for the criminal he is. In fact, it was not uncommon at certain points for her to be billed above her mistress, the play's ostensible heroine.
In the original story on which Othello is based, the Moorish character doesn't even have a name, and it ends with Desdemona lecturing the audience on why interracial marriage is evil. In his adaptation, Shakespeare gives the Moor a name and fully fleshes out his character into a sympathetic war hero. In fact, the only overtly racist elements of the play are spoken by unsympathetic characters.
In regards for how the play treats women, it's not quite as cut-and-dry as one may think. It is true that the play revolves around Slut-Shaming, and Emilia tolerates a lot of horrific treatment from her husband (and later, after Othello goes full Green-Eyed Monster, Desdemona does as well). However, Emilia is remarkably resilient and proactive, and is the one who ensures Iago gets his just deserts at the end, and Desdemona isn't quite the one-dimensional pushover people remember. (She's admittedly quite placid, but she also willingly went behind her racist father's back to marry the man she loved, which is a rather gutsy move.) It's also pretty clear that the aforementioned slut-shaming is wrong; it ultimately gets an innocent woman killed, and Cassio's poor treatment of Bianca (a perfectly nice courtesan he's been seeing) solidifies him as a bit of a jerkass. Emilia also has a rather impressive monologue where she says that, yes, some women do cheat, but we shouldn't be so quick to judge them, since men cheat, too, and many women are driven to cheat because their husbands abuse them. So while the play isn't exactly pro-feminist, it's still got some interesting points where the treatment of female characters are concerned.
Ho Yay: Some scholars — and many a high school English teacher — have proposed that Iago's true motivation is unrequited love for Othello. You can find evidence for this in his dialogue.
Then again, you can find evidence for at least a dozen, often conflicting, motivations in his dialog. He also says he has no reason at all. See also For the Evulz.
Might be a case of a deliberate Multiple-Choice Past, to emphasize that Iago is an unrepentant liar..
For some Iago's description of Cassio in bed with him and "sleep humping" might fit this.
An oft-cited piece of evidence for this possible motivation is the scene wherein Iago and Othello initiate a pseudo-wedding ceremony. To each other.
Othello:*kneels* In the due reverence of a sacred vow I here engage my words. Iago: Do not rise yet. *kneels* Witness, you ever-burning lights above, You elements that clip us round about, Witness that here Iago doth give up The execution of his wit, hands, heart, To wrong'd Othello's service! Let him command, And to obey shall be in me remorse, What bloody business ever. *they rise* Othello: I greet thy love, Not with vain thanks, but with acceptance bounteous, And will upon the instant put thee to't: Within these three days let me hear thee say That Cassio's not alive. . . . Iago: I am your own for ever.
John Quincy Adams: "Who can feel sympathy for Desdemona? A woman who, born and educated to a splendid and lofty station in the community, betrays her race, her sex, her duty and her country, and makes a runaway match with a blackamoor!"
Desdemona having a few last words after being strangled to death. Considering how hard it is for death by strangulation to work, it's perhaps a case of Reality Is Unrealistic, and her final words where she refuses to blame Othello for her death, elevates her above someone fridged.
In the Olivier film, the make-up used to make Olivier look black more often makes him look blue or grey.
Nightmare Fuel: The whole show, really. So many lives are ruined by one man's hatred. Iago sets off the chain of events that does all this damage with a well-placed handkerchief, and rumors. Over the course of less than a week, one man manages to drive a previously devoted husband to murder his wife, and indirectly causes the death of four others, without a hint of remorse. And we don't even really knowwhy. The best we can figure is that he just thinks it's fun.
Paranoia Fuel: There is something deeply unsettling about the character Iago. The idea that someone you trust implicitly could be so sociopathic that the first minor, unintentional sleight you perpetrate against them could lead them to utterly destroy your life for kicks is very creepy. Stabbing his own wife without the slightest hint of regret or reluctance.
Rescued from the Scrappy Heap: As feminism began to seriously kick off in the twentieth century, new attention was paid to the traditionally-disdained character of Desdemona. The especial turning point is usually seen as Maggie Smith's tough, angry Silk Hiding Steel performance in 1964, which encouraged later productions to put much more fight and sexuality in their Desdemonas after a long period of passive, chaste bowdlerisation. Even the more demure versions since then have become more prominent and central more often, with their tragedy being emphasised rather than being sidelined to better serve the interplay of Othello and Iago.
The Scrappy: Starting in the eighteenth century, Desdemona's character was heavily bowdlerised to remove the more subversive elements and make her nothing other than a demure, passive, innocent victim with all the personality of a damp piece of paper. This characterisation stuck for most of the next 300 years, and made her exceedingly unpopular amongst both actors and the audience. The actor William Macready warned Fanny Kemble thusly before she took on the role (and actually turned out to be one of the more successful and spirited Desdemonas of the time):
"... there is absolutely nothing to be done with (the part of Desdemona), nothing; nobody can produce an effect in it; and really, Emilia's last scene can be made a great deal more of. I could understand your playing that, but not Desdemona."
Values Dissonance: Some readers won't understand that when Othello gives up Christianity, he super-damns himself to Hell; that's even worse than just being a pagan.
More obviously: these days, a black man marrying a white woman would not raise many eyebrows. Back then, not so much.
Values Resonance: The play's main message (interracial marriage can be a loving one and racism can bring severe consequences) is still, if not more, relevant in today world of global traveling easier than Shakespeare's time. Likewise, relationships are difficult and messy, and the play's rather overt criticism of the MadonnaWhore Complex, and its portrayal of a loving husband can become domineering and abusive, and make it hard for the wife to resist, makes it a pretty realistic work.
Vindicated by History: Even more than usual for Shakespeare - he subverted a lot of Forgotten Tropes at a time when a Cliché Storm was expected. Thomas Rymer's Short View of Tragedy in 1693 summed up the response to, for instance, a soldier as a villain rather than an honest man, and a dropped handkerchief leading to multiple murders rather than a comical misunderstanding.
Desdemona has a very, very bad week. She just wants to spend some time with her husband, and for her trouble, gets Mistaken for Cheating, humiliated, abused, and ultimately smothered in her own bed. And she dies refusing to name Othello as her killer.
Othello himself, sometimes bordering on Jerkass Woobie, depending on the actor. Yeah, he's got a jealous streak a mile wide, and he can be a bit self-centered, but the guy's obviously got some internalized racism, as well as some major insecurities in his marriage to Desdemona, unable to see that she really does love him. Throw in a Manipulative Bastard and you've got a recipe for a tragedy. Specifically, this one.
Emilia, Iago's wife, mostly on the grounds of being Iago's wife. She tries to please him, to no avail, and one of these attempts ultimately gets her best friend murdered. And then when Emilia finally outs Iago as the bastard he is, he kills her, too.
Anvilicious: Almost all chapters transmit the message that bullying is bad and that you should stand against it. Also, one chapter is centered exclusively on reporting train sexual harassers while barely apporting to the plot itself.
Catharsis Factor: Nana while dancing on the school roof beats up all of Megumi's henchmen, before stealing a skateboard to chase Megumi.
The Woobie: Yaya. She lives without a mother, her father is too strict with her, she's never had any friends and her only emotional support is following a (already disbanded) Visual Kei band. Wow.