Iago seeks to destroy his superior, the Moorish general Othello, both personally and professionally by convincing him that his loving wife is actually unfaithful. Iago can't decide on why he's getting revenge, since he offers up several conflicting rationalizations. Ultimately, Iago destroys Othello simply because he doesn't like him.
He did it because Othello was going to make someone his lieutenant... and that someone wasn't Iago.
Still definitely Disproportionate Retribution. He ruins Cassio's reputation and tried to have him assassinated, mentally destroys Othello and causes him to murder his wife, then kill himself. Because he was passed over for a promotion.
That motive isn't any more plausible than any of his other ones, though.
In Macbeth, one of the witches causes a sailor to be shipwrecked tossed about in a horrible storm... because his wife wouldn't give the witch a roasted chestnut. (The text explicitly says that the boat won't sink, possibly due to some kind of limitation on the witches' powers, but the sailor will have a thoroughly miserable time of it.)
One could argue that both Shylock's "pound of flesh" contract and the punishments inflicted upon him at the end of the play are both examples of this trope.
The title character in Phantom of the Opera murders stagehand Joseph Buquet when the opera managers refuse to give his beloved Christine the lead role in the opera, despite the fact that they stopped the show and made a point of announcing that the pause was so that Christine could step into the part. Later, incensed at Christine's betrayal (with absolutely zero thought to the fact that his behavior has terrified her), he drops a massive chandelier into the audience, where it would have likely injured or killed numerous people, including Christine.
Liz: So, I said to him, I said, "You pop that gum one more time..." And he did. So I took the shotgun off the wall and I fired two warning shots...into his head!
At Act I, Ligniere exposes Count De Guiche plan to bully Roxane into being The Mistress. De Guiche sends a hundred men to punish Ligniere. And from here, It Got Worse. De Guiche ends with power and influence only to discover that is Lonely at the Top.
At Act I Scene IV, Cyrano destroys Montfleury’s delusions of glory and love because he is a bad actor and interrupts La Clorise because the verses of his author, Baró, are worth nothing. Later, when a Bore bothers him, he invokes this trope using his nose so he can literally kick the Bore’s ass. Hilarity Ensues. From here, It Got Worse. Cyrano ends his days in Perpetual Poverty (because his Pride is so big he would not left his friends help him).
Handled hilariously in As You Like It, when Touchstone finds out that someone else (a simple, non-threatening peasant) likes Audrey: "...abandon the society of this female, or, clown, thou perishest; or, to thy better understanding, diest; or, to wit I kill thee, make thee away, translate thy life into death, thy liberty into bondage: I will deal in poison with thee, or in bastinado, or in steel; I will bandy with thee in faction; I will o'errun thee with policy; I will kill thee a hundred and fifty ways: therefore tremble and depart!" Especially disproportionate since Touchstone isn't really all that into Audrey and is only marrying her so that he can justify having a fling with her.
