You might think of Iago as a sort of puppet master, one who works behind the scenes, unseen, and pulling the strings. Envious of higher positions in Othello's army (he's an ensign), outwardly he is "a man...of honesty and trust" but actually he is a villain without a conscience; a "viper" and an "inhuman dog."
His dual nature is first realized in Act One, when Iago's first plot against Othello is to tell her father about his daughter's elopment with the nobleman. He brings another puppet onstage, Roderigo, to take the fall.
Often, Iago (and the play) is helped along with his wicked schemes by fortunate circumstance. For example, Othello chooses Iago to accompany Desdemona; therefore, Iago has the opportunity to observe her with Rodergio and come up with the material needed to construct a story of infidelity. Additionally, along the "luck" theory, Iago comes upon Desdemona's handkerchief, which he uses against her as "evidence."
The play ends with Iago's punishment, his letters of betrayal found in the coat pockets of the slain Rodgerio. Iago stays a force in the play until the very last lines, when "the Spartan dog" is hauled off on Cassio's orders. He will be tortured and murdered for his crimes.