"Iago is Othello's sword-bearer. He has been passed over for the position of Lieutenant, and this draws out his evil nature. He feels that he has been wronged and cannot accept the position that Othello gives him. Iago orchestrates Othello's downfall out of malice and revenge, and it is worse because Iago coats his poisonous words in the appearance of truth. Othello has no reason to doubt Iago, and Iago is able to completely manipulate Othello and to control his actions. Othello kills Desdemona, but Iago is responsible. Iago is only concerned about himself and his position, and will sacrifice anyone to save himself and his interests. This is shown when he kills his own wife when it is discovered that he has been treacherous. "( http://www.novelguide.com/othello/characterprofiles.html )
At the start of the play, Iago and Rodrigo are talking about how Desdemona eloped with Othello. They are on their way to report this to Desdemona's father. Rodrigo criticizes Iago for not helping him wed Desdemona, and makes it seem that it was Iago's fault that Othello married Desdemona. Iago assures Rodrigo that he is not that loyal to Othello because Othello didn't promote him.
Later at the Venetian council Othello is accused of putting a spell on Desdemona. When Desdemona says she is loyal to her father and to Othello, Othello is told to prepare for war. This is where Iago starts to plan his revenge.
A few weeks later on the island of Cyprus, Governor Montano greats Cassio, who has just arrived from war. Later, Iago, Emilia, and Desdemona arrive. With the warm welcome of Desdemona from Cassio, Iago decides to use him in his revenge on Othello. Iago then begins to convince Rodrigo that Cassio is...