Iago carefully maintains a veneer of "honesty and trust" but like many people, his outward appearance belies a inner deception. Iago is commonly referred to as Machiavellian, a term coined for Prince Machiavelli. Machiavelli is famous for his political treatise, "The Prince" which espouses, among other things, that the ends to power always justify the means.
Othello is particularly susceptible to Iago's ploys. Iago himself is so noble and unsuspecting of villainy that he easily plays into Iago's hands.
Iago will use anyone and anything to get what he wants. He uses Rodergio, for example, encouraging the young man to continue his pursuit of Othello's wife. He then plots to make Othello believe that Desdemona has been unfaithful with the lieutenant. Iago plies Cassio with wine, then encourages him to pick a pick a fight with Rodergio. "In his shame," says critic Homer Watt, "Cassio falls readily into Iago's suggestion that he beg Desdemona to intercede on his behalf. It is the villain's trap."
By constructing situations in which Iago appears blameless and other fall, this cold, egotistical man stays above suspicion.