Othello really thinks he's talking about Desdemona...
(Bianca) loves me so much!
Iago tells Othello that Desdemona is allegedly cheating on him with Cassio. This conflict is external because he believes Iago and was manipulated by him. Othello was shocked and did not think his wife would actually cheat on him. For example, in Act III Scene iii Iago says, "Watch how she is with Cassio... They let God see things they wouldn't show their husbands .They don't avoid doing things that are wrong , they just try not to get caught." Since Othello is gullible enough to believe "Honest Iago," he is full of rage and wishes to prove the affair.
Othello hides as Iago and Cassio have a conversation. He overhears Cassio talking about sleeping with someone , and he believes it is Desdemona. Cassio is really talking about Bianca, his jealous lover. Othello feels the reason he's been cheated on is that he isn't good enough for Desdemona, and he feels insecure about himself. Act III Scene iii states,"Maybe because I’m black, and I don’t have nice manners like courtiers do, or because I’m getting old." Othello started to blame himself for Desdemona's alleged affair with Cassio. This is the effect of Iago trying to mess with Othello's brain.
Othello slaps Desdemona when she repeatedly compliments Cassio and praises him for his new rank as the leader in Cyprus. Othello, who just pictured her and Cassio sleeping with each other, has enough of her comments and slaps her. In Act IV Scene i, Desdemona says, "Yes, unfortunately. I’d do anything to bring them back together, since I really care for Cassio." Then, Othello says, "I’m glad you’re insane enough to admit it in front of me...(striking her) You devil!" The "affair" and Desdemona's comments about Cassio caught up to Othello and he took it out on Desdemona.