How does Shakespeare's tragedy Othello explore aspects of human nature?

Othello, like all of Shakespeare's works, shows an acute understanding of people and what drives them either to good or evil. Within the context of the plot, it is Iago, of course, who thinks himself the puppeteer of the other characters, utilizing their anticipated responses to drive his scheme onward, but in the end it is often Iago's motivations which fascinate and preoccupy us.
Iago's great strength is his understanding that human nature is not identical from person to person. He is able to progress his scheme because he understands that people are different and react in different ways to different stimuli. Othello, an insecure and jealous man who is already in an insecure position in society because of his race, can easily be manipulated when Iago preys upon his insecurities, particularly because he loves his wife so much. Othello's love, coupled with the jealousy Iago stirs in him, quickly turns to jealousy and anger. Meanwhile, Desdemona, who is loyal, sweet, and generous, can easily be prevailed upon to petition Othello on behalf of Cassio, which of course leads into Othello's jealousy and insecurity.
Cassio, a good man, has one obvious weak spot: he cannot hold his alcohol. Iago takes advantage of this and causes him to lose his "reputation," which he valued above all else. Roderigo loves Desdemona and is also fundamentally weak; he goes along with Iago's plan because he wants to "enjoy" her, whatever the cost. Emilia surprises Iago by demonstrating such great loyalty to her mistress at the end, such that Iago is exposed and found out.
But what is Iago's nature? What drives him? This is never really made clear in the play. He says that he does "hate the Moor" because he has elected the unqualified Cassio as his lieutenant over Iago; he also says that he fears Othello has "leaped into my seat," or had sex with his wife. But these motivations never seem to tell the full story. In the end, Iago's understanding of all the other characters' motivations is what drives the play, but his own motivations remain shrouded in darkness.