Shakespeare presents Iago as a collection of unsolvable puzzles. Each thing Iago says is cause for worry. He claims a reputation for honesty and plain speaking, yet he invents elaborate lies in order to exploit and manipulate other people. He treats others as fools and has no time for tender emotion, yet he is a married man and presumably once loved his wife. He cares for no one, yet he devotes his whole life to revenge rather than walk away in disdain. He believes in cheating and lying for gain, yet Shakespeare placed some of the most beautiful words in Iago's mouth.
Iago has a reputation for honesty, for reliability and direct speaking. Othello and others in the play constantly refer to him as "honest Iago." He has risen through the ranks in the army by merit and achievement, and Othello, whose military judgment is excellent, has taken him as ancient (captain) because of his qualities. In Iago, Shakespeare shows us a character who acts against his reputation. Possibly Iago was always a villain and confidence trickster who set up a false reputation for honesty, but how can one set up a reputation for honesty except by being consistently honest over a long period of time? Alternatively he might be a man who used to be honest in the past, but has decided to abandon this virtue.
Shakespeare has built the character of Iago from an idea already existing in the theatrical culture of his time: the Devil in religious morality plays, which developed into the villain in Elizabethan drama and tragedy. Iago says (I.1, 65) "I am not what I am," which can be interpreted as "I am not what I seem." But it is also reminiscent of a quotation from the Bible which Shakespeare would have known: In Exodus, God gives his laws to Moses on Mt. Sinai, and Moses asks God his name. God replies: "I am that I am" (Exodus,iii,14). If "I am that I am" stands for God, then Iago's self-description, "I am not what I am" is the direct opposite. Iago is the opposite of God, that is, he is the Devil. Iago in this play, has the qualities of the Devil in medieval and Renaissance morality plays: He is a liar, he makes promises he has no intention of keeping, he tells fancy stories in order to trap people and lead them to their destruction, and he sees other's greatest vulnerabilities and uses these to destroy them. Iago does all this not for any good reason, but for love of evil.
Iago is surrounded with bitter irony: he is not as he seems, his good is bad for others, people repeatedly rely on him, and he betrays them. He likes to have others unwittingly working to serve his purposes. But for all this, as his plot against Othello starts moving and gathering momentum, he loses control of it and must take real risks to prevent it from crashing. Iago is a man with an obsession for control and power over others who has let this obsession take over his whole life. Necessity forces his hand, and, in order to destroy Othello, he must also destroy Roderigo, Emilia, Desdemona, and ultimately himself. The one man who survived Iago's attempt to kill him, Cassio, is the only major character left standing at the end of the play.
William Hazlitt wrote: "Iago is an extreme instance . . . of diseased intellectual activity, with the most perfect indifference to moral good or evil, or rather with a decided preference of the latter, because it falls more readily in with his favorite propensity, gives greater zest to his thoughts and scope to his actions. He is quite or nearly indifferent to his own fate as to that of others; he runs all risks for a trifling and doubtful advantage, and is himself the dupe and victim of ruling passion — an insatiable craving after action of the most difficult and dangerous kind."
The great nineteenth-century actor Booth wrote about playing Iago: "To portray Iago properly you must seem to be what all the characters think, and say, you are, not what the spectators know you to be; try to win even them by your sincerity. Don't act the villain, don't look it, or speak it, (by scowling and growling, I mean), but think it all the time. Be genial, sometimes jovial, always gentlemanly. Quick in motion as in thought; lithe and sinuous as a snake."