Might Iago be envious of Othello because he receives affection from Desdemona, when Iago and Emilia don't connect?
I know that Iago is envious over Cassio being placed as lieutenant. Iago also suspects Othello and Cassio of sleeping with Emilia. But I feel it could possibly be that Emilia and Iago do not have a connection. This could also be the reason for his jealousy over everyone else in the play. That could also by why after Othello starts doubting his wife Iago switched his motivations to Cassio for having a relationship with Bianca.
I think it's certainly a possible interpretation. Desdemona and Othello are hugely affectionate - not least in the presence of Iago and Emilia when the army come aboard from the journey to Cyprus.
O my fair warrior!
My dear Othello!
It gives me wonder great as my content
To see you here before me. O my soul's joy!
If after every tempest come such calms,
May the winds blow till they have waken'd death...
If it were now to die,
'Twere now to be most happy; for I fear
My soul hath her content so absolute
That not another comfort like to this
Succeeds in unknown fate.
And the very little we see of Emilia and Iago together seems to suggest an extremely unloving, unwarm relationship (look, for example, at the way she gets the hankerchief to try and get his attention - but is simply told to go in once he has got it!). Emilia keeps saying things about how awful husbands are: this quote suggests Iago uses her simply for sex:
They are all but stomachs and we all but food;
They eat us hungerly, and when they are full
They belch us.
It's a good interpretation. Though, of course, we can never be sure about Iago's motives. His deliberate silence ("what you know you know") at the end of the play points to Shakespeare's intention: it's awful, but we never know why it has to happen.
I know what you mean about Iago - but I don't think he is a stereotype or evil for evil's sake (though Coleridge famously agreed with you, ascribing to him a "motiveless malignity").
I can't think of any other characters, in Shakespeare or elsewhere, that are like him - he gives us several reasons early on for what he is doing, and then taunts the audience by not revealing his plan ("I have it. 'Tis engendered" he says at the end of one soiloquy). So he has too many reasons for his behaviour - and we don't believe any of them... or all of them.
If you believe that no-one is born bad, then you have to conclude that Iago, far from being a flat character, is a terrifying puzzle.
Iago is a nasty guy who doesn't really need a reason or justification for his rage against Othello. However, he doesn't seem to be that personally invested in his relationship with his wife, and his priorities seem to rather lie in the prestige of his post and career advancement.
Iago might be shrewd and cunning, even perfidous in the pleasure he gets from hurting others, but his character profile is not that hard to figure out. He is the antagonist of the story, he is evil for evil's sake, and he is a flat character (even a stereotype) who does not change one bit throughout the story line.
What do you mean by flat character of Iago?
But although he doesn't invest much into his relationship with his wife, he does get much out of hearing about other's relationships. He is obsessed with that.
I agree with robertwilliam- I think it is not that he is all evil, I think that he has many motives and this could just be yet another one.