In 'Othello' the central theme is jealousy and betrayal. Iago inspires jealousy in several of the central characters, most notably, Roderigo and Othello.
Iago sets in place malice in Roderigo towards Othello for marrying Desdemona but what Roderigo does not realise is that he is being duped by Iago who not only has no intention to promote Roderigo to Desdemona but is also milking Roderigo for every penny that he is worth.
Iago plants seeds of doubt about Desdemona's faithfulness to Othello informing him that she has been unfaithful to him with Cassio and as a result he murders her and asks Iago to kill Cassio on his behalf.
Eventually, all parties come to realise that Iago has betrayed them all.
He betrays Othello by lying about Cassio and Desdemona
He betrays Roderigo by lying about Cassio and Desdemona and subsequently killing him.
He betrays his wife by killing her when she exposes him.
He betrayed Desdemona by lying about her virtue.
He betrays Cassio by engineering his demotion.
Othello is not innocent:
He also betrays Desdemona
He betrays Cassio
and to a certain extent he betrays himself by succumbing to jealousy and fulfilling the expectations that Venetian society had of him based on race.
He also betrays the audeince because in the final Act when we think he is going to come clean and explain his actions he does the opposite and decides to remain silent.
If you are looking for key quotations, Act 3 Scene 3 is the central scene and marks the turning point of the play. In this scene, there are several quotations that relate to jealousy and describe it as being a poison or disease. In terms of betrayal the final Act provides several good examples of how Othello copes with the realisation of his betrayal of Desdemona.