Firstly, he admits it! In 4.1, he states "I am a very villain else."
Unlike villains who "know not what they do," Iago is in full awareness of the results of his actions. He knows what he does is wrong, but he doesn't care -- he is after authority and power, and as we find out in the course of the play, the kingship itself, and he will stop at nothing to achieve his goal. What appears to motivate him is sheer hate.
One of his villainous techniques is compartmentalization -- telling one person or persons one thing, and another person or persons something else. He can then play one off against the other, and by controlling the situation, implement his schemes. In 2.3, he advises Cassio to ask Desdemona to have Othello reinstate him; Iago soliloquizes about how he will use that to manipulate everyone to his own purpose, "enmesh them all." He'll tell Othello that Cassio is seeking Desdemona so he can sleep with her, this will enrage Othello, discredit both Desdemona and Cassio, and put Iago in control.