Firstly, love. To Othello, love needs passion and trust to maintain. As a black man, even if he was the general, falling in love with Desdemona goes against the traditional 16th century Venice notions. They fell in love just because of passion, with Desdemona disobeying her father and not considering the importance of filial obedience. “O heaven! How got she out? O treason of the blood!” shows how angry Brabantio, Desdemona’s father is when he found out Othello and Desdemona’s relationship. He could not believe that his own blood and flesh betrayed him.
Othello also has trust. He believes in their love. “She loved me for the dangers I had passed, / And I loved her that she did pity them. / This only is the witchcraft I have used.” indicates that he is confident that people around them will trust in their true love as well.
On the other hand, Iago sees love as leverage, which he can use for his own good. He uses Othello and Desdemona’s love to ruin Othello’s success and both of their reputations. Iago catches Brabantio’s attention and exposes their relationship by calling out, “Thieves, thieves! / Look to your house, your daughter, and your bags! / Thieves, thieves!” He also uses Desdemona's love in friendship for Cassio to make her appear to be disloyal. “Be thou assured, good Cassio, I will do / All my abilities in thy behalf.” is where Desdemona promises to help Cassio. Iago also took advantage of Cassio’s courtesy of “takes her by the palm”.
He persuades Cassio to have interaction with Desdemona by saying, “Our general’s wife is now the general.” With Cassio and Desdemona’s interaction, Iago convinces Othello that they are having an affair. “She did deceive her father, marrying you;” is when Iago pointed out Desdemona once lied to his father, so she may betray Othello too.
Secondly, appearance versus reality. Othello believes that evidence will prove the truth. He stays calm when others accused him of his love story with Desdemona as he knows that she will speak the truth. His evidence for everyone is “I do beseech you / Send for the lady to the Sagittary / And let her speak of me before her father.” and “Here comes the lady: let her witness it.”
However, for Iago, due to his loss of moral compass, what he desires, and imagines is the truth instead. He then spreads his own imaginations to lead others into believing his lies. “And what’s he then that says I play the villain, / When this advice is free I give, and honest,” and the repetition of “How am I then a villain,” shows that Iago cannot distinguish between good and bad.
Christianity was the main religion at that time. Affected by the Great Chain of Being, Iago is envy of Othello’s power, status and position as a general. All of his lies come from this jealousy, he begins to plot to take revenge. As a part of his plan, the person he portrays to everyone else is not who he really is, as shown in “I am not what I am.”
Iago with his manipulative nature, tells Othello to imagine Desdemona and Cassio together: “What, / to kiss in private?” “Or to be naked with her friend in bed / An hour or more, not meaning any harm?” With these imaginations, Othello’s jealousy has ramped up already.
Iago fails to understand Othello’s view and values, insisting that everything he did is for a good deed. Eventually, Othello fell for his lies, resulting in his own downfall and the tragedy of the whole play.