Iago is a jealous, manipulating, vindictive type character. He wants what Cassio has. He desires to be in command as Cassio is. For this reason, he seeks to destroy Cassio's reputation. Since Cassio is of reputable character, Iago has to get Cassio drunk to taint his reputation.
Iago will stop at nothing to ruin the lives of those around him. He is power hungry. He desires to be a position of power. He is a controlling sort of man. Iago goes to great lengths to destroy Cassio. He uses Roderigo to move his plan forward. Iago does not care about anyone but himself.
Iago befriends Roderigo only to use him in the end. There is absolutely nothing honorable about Iago. He doesn't even love his own wife. He uses her to execute his plan to destroy not only Cassio but also Othello. When Iago's wife gets in the way, Iago kills her. He is a deceptive man from the beginning to the end.
Iago is an excellent actor. He deceives Othello. Othello believes every word that Iago speaks. Iago is sharp witted. He is conniving. He manages to deceive Othello while pretending to be loyal to Othello. Iago is a lying, deceiving man:
[Iago] is also one of Shakespeare's greatest villains. He is a master manipulator of people and gets the other characters in the play to do just what he wants. He manipulates others through a keen understanding he seems to have of what motivates them.
Iago uses Othello's jealous nature to set up Desdemona. He convinces Othello that Desdemona is unfaithful:
And, finally, Iago uses Othello's jealous nature and his apparent insecurity to convince Othello of Desdemona's infidelity.
No doubt, Iago is evil. He is a reprehensible character who uses and manipulates anyone in his way. Shakespeare created a most evil villain when he created Iago:
But more than any other character in Shakespeare's plays, Iago is a self-professed villain whose sole motive is hatred toward his superior. It is the unrelievedly evil, maniacal fixation of Iago that provides such lines as his off-hand comment, "I am a very villain else" (IV.i.126), with their acute pungency.