Iago is seen as honest. Therefore he must have been an honest person before he decide to ruin Othello and Cassio, and this proves that he was not always a villain.
Iago has been honest his entire life, and it is getting him nothing. So he tries to get the life he wants by taking revenge against Othello and Cassio. Iago feels honesty is getting him nowhere, so he turns evil. Iago begins to search for motives so he can rest his conscience from the evil he is going to do. In Iago's numerous soliloquies, he comes up with lots of possible motives for his plot. Iago first says his motive for hurting Othello is that he promoted Cassio and not himself. And after the first soliloquy Iago never mentions it again. In the fifth act Iago states: Cassio "hath a daily beauty in his life that makes me ugly" [V. i. 19-20]. So he then starts to blame Cassio. But Iago has already been made lieutenant at this point in the play, so he has some other grudge against Cassio. Iago also suspects Othello and Cassio of being sexually involved with his own wife. Iago has no evidence of this and never mentions it again. He also can not be that upset about his wife having an affair, because he shows no affection for her and ends up killing her. Iago also mentions that he loves Desdemona in the second soliloquy, but does not mention it again. Iago has turned evil to get his life back on track, but shows some conscience in doing so. In Iago's first soliloquy he says when talking about Othello and Emilia, "I know not if't be true, but I, for mere suspicion in that kind, will do as if for surety." [I. iii. 358-360]. .
One of the reasons Iago is seen as such an evil character is because he is so intelligent. An evil person with intelligence can be a very dangerous villain. Iago knows exactly what to say to every character that makes their jealousy come to life. He knows exactly where everyone's weakness is, and he knows exactly how to make the jealous rage unfold in each character.
