Many observers have tried to work out Iago’s motivation as if they are mystical and arcane. However, it is easy to see why Iago wants revenge on Othello.
Revenge
After years of loyal service, Iago has been passed over for promotion.
The ‘counter-caster’ Cassio is favoured; a man who ‘never set a squadron on the field’.
Class-hatred
You could argue that Iago is motivated by class-hatred.
Some critics think that Iago has working-class origins and detests favour and promotion by reputation alone.
Racism
You could argue that Iago has strong racial prejudices based on the racial slurs he makes about Othello.
Infidelity
Iago also has his suspicions about Othello having had a relationship with Emilia, Iago’s wife.
Iago's Nature
While other characters alter and progress as the play develops, Iago stays very much the same. He does not evolve.
Manipulation
Iago is a strategist and is able to move pieces around a chessboard as if he is an expert player.
He is very aware of his own lies and enjoys setting them into place.
The audience knows this both from his asides and his soliloquies.
Iago's lies
Iago wears a mask throughout (‘I am not what I am’).
He first lies to Brabantio about his daughter, saying that Othello has bewitched her, and then lies to Roderigo saying that Cassio is a drunkard.
He tells Othello that Desdemona gave Cassio the handkerchief.
Iago'scunning
The way Iago works is to use his victims’ noble natures.
All the time he pretends to be virtuous and sensible so that the other characters trust him.
Soliloquies
The evil nature of Iago comes out in his soliloquies where he confesses his hatred of others and allows the audience to see his mind-set.
His villainy remains even to the end, where he will not confess the reason for his actions.
Iago: Love and Tragedy
You could argue that Iago contributes to the love strand of the play by erecting obstacles for the lovers to react to and overcome, and while he is not the cause of the tragedy, he is the one who sets the traps.
Performance
Sometimes, the actors playing Iago and Othello swap roles every other night, to show that they are a kind of double act, with one representing light (Othello) and the other offering darkness.
However, Othello aims to be noble, and Iago is very different: he is a base kind of character and such a performance technique may weaken the tragic hero position of Othello.
Iago and love
You could argue that Iago contributes to the love strand of the play by erecting obstacles for the lovers to react to and overcome.
His wish for revenge on Othello is hugely motivating and this ends up destroying much of the love in the play.
His actions even destroy his own relationship and marriage to Emilia.
Iago and tragedy
Iago is perhaps not the cause of the tragedy because that itself is found in the blindness of Othello, but Iago is the one who sets the traps.
He is therefore a classic tragic villain.
The play is unusual in that in most tragedies the tragic villain is killed, but here, he does survive, perhaps to face interrogation and torture over his actions.