Iago is unabashedly obsessed with revenge, and is one of the few characters in Shakespeare who, when given the opportunity to relent and seek forgiveness in the end, refuses to seek absolution, even as he lay at othello's feet.
In essence he personifies naked evil, and revels in his status as one who would do anything to get what he wants, without apology. often, Shakespeare makes "bad" characters have reasons for their behavior, and we may gain a better understanding of the complexity of the character's bad behavior. For instance, Tybalt, in Romeo and Juliet, hates Romeo because his family has been feuding with Romeo's for generations, and feels slighted by Romeo's perceived "flaunting" of his presence at the Capulet party. Shakespeare never fully and completely explains Iago's obsession with revenge; Iago makes a small remark about rumor that the Moor had slept with Iago's wife, but then never mentions it again; he talks of being slighted by Othello whenit comes to promotions, but it appears that Iago is still held in high esteem, despite Iago's assertions; and, of course, there is a racial hatred, and disgust for Iago's interracial marriage (and perhaps a little jealousy over Othello's landing of such a beautiful wife, to boot). But all of these ideas are given short shrift until Iago moves to the next excuse. We see iago as an irrational, thoroughly evil man who has an unnatural obsession with revenge. This works because we can watch Iago's plans and marvel at their evil ingenuity, without having feelings or emotions toward him.