The Theme in the Work
What the play shows first is resentment, and Cassio's envy appears only at the end. In scene 1 of Act I, Iago tells Roderigo that he hates Othello for giving the lieutenancy to Cassio, and that he serves him only out of convenience: the resentment of one who was passed over comes before any proof, any accusation, any plan. In scene 3 of Act I, the resentment becomes a plan: Iago convinces Roderigo to sell his lands and follows to Cyprus, and he formulates what he will exploit in Othello.
In scene 1 of Act V, envy appears explicitly in Iago's calculation in an aside: he says that the death of either of the two suits him, and that, if Cassio lives, he has in his life an ever-present beauty that makes him uglier. In scene 2 of Act V, the outcome is that only Cassio, the original target of Iago's resentment, comes out alive and in command of Cyprus.
Iago's envy is seen best in the qualities of Cassio that he chooses to attack. In scene 1 of Act I, he describes Cassio as an "arithmetician," a theorist of books who never led a squadron in the field, and compares himself to him, who fought beside Othello at Rhodes and in Cyprus. In scene 1 of Act II, he tells Roderigo that Cassio is dashing, young, and has the manners and polish that would please Desdemona, everything, he says, that the Moor lacks; and, in an aside, he sees Cassio kiss Desdemona's fingers in courtesy and promises to entangle him in those same courtesies. In scene 3 of Act II, Iago praises Cassio aloud, as a soldier worthy to stand beside Caesar, only to say at once that his vice cancels his virtue and may shake the island. Envy, thus, almost never speaks in its own name: it appears as praise that serves as accusation.
Iago accumulates motives without settling on any: the post given to Cassio and hatred of the Moor (I.1, ll. 7-33), the suspicion that Othello lay with Emilia (I.3, ll. 391-394; II.1, ll. 295-299) and, at the end, Cassio's ever-present beauty (V.1, ll. 18-20). The first motive is being passed over, and envy appears only last, but Cassio is the target of both.
Where It Appears
- Scene 1 of Act I: Iago tells Roderigo that he hates Othello for giving the lieutenancy to Cassio and that he serves him only out of convenience.
- Scene 3 of Act I: At the end of the act, Iago, who hates Othello for giving the lieutenancy to Cassio, convinces Roderigo to sell lands and follow to Cyprus, using the money only as bait.
- Scene 1 of Act V: In an aside, Iago admits that the death of Roderigo or of Cassio suits him, Cassio being the original target of his resentment, and concludes that Cassio must die.
- Scene 2 of Act V: At the end of the play, Iago is captured, and Cassio, the original target of his resentment, survives the ambush and remains in command of Cyprus.
Passages
Iago explains to Roderigo why he serves Othello and closes with a warning about himself:
For daws to peck at. I am not what I am.(I.1, l. 64)
As he closes the plan, Iago already thinks of it as something begotten:
I have 't. It is engendered. Hell and night / Must bring this monstrous birth to the world's light.(I.3, ll. 408-409)
In the ambush, envy calls for death:
No, he must die. Be't so.(V.1, l. 22)