Othello is a very noble person at the start of the story. He respects others and is confident in his abilities. He is also very courage's and will put his life on the line to prove/ calm down something. For example, in Act 1, Scene 3, Pg 6, "If she [Desdemona] has anything bad to say about me, then you can sentence me to death". This shows he's a very confident person and is noble but is all about to change.
Othello starts to lose his composure when he finds out that a drunk Cassio had started a fight with Montano, a very wise and respected person in Venice. He shows a hint of anger when he demands what caused the situation. For example in Act 2, Scene 3, Pg 9, "All right, now I’m starting to lose my cool. By God, if you don’t tell me what happened you’ll all suffer.
Othello is still upset at Cassio after the whole ordeal is done because when Cassio tries to make the general happy with some music being played by a couple of musicans, Othello sends someone to tell them to stop. In Act 3, Scene 1, "The general likes your music a lot, but he asks you to stop playing now."
Othello starts to become really angry and starts to act delusional after hearing Iago telling him that Desdemona is cheating on Cassio. This rubs him in the wrong place really badly because hes been seeing Desdemona hang out a lot with Cassio and it hurts him to think that Desdemona would cheat on him and thus actually plans to kill her. In Act 4, Scene 1, Pg 9, "Yes, let her die and rot and go to hell tonight. She won’t stay alive for long." After this he decides to strangle her to death and when she comes into his room later when attendants from the Duke he slaps her and she runs off.
When Othello finds Desdemona sleeping in their bed, he starts talking about how he's going to kill her. Desdemona pleads with Othello to give her at least have an hour but he refuses and straight up strangles her to near death and later dies on her own. In Act 5, Scene 2, Pg 5, "OTHELLO-Now, if you struggle with me. DESDEMONA-Just half an hour! OTHELLO-While I’m doing this, I can’t pause for a moment. DESDEMONA-Just let me say one prayer! OTHELLO-It’s too late. (he proceeds to smother her)."
When Othello finally finds out that Iago tricked him intro thinking Desdemona had cheated on him he has him and Cassio be brought back to the room where Desdemona was killed. Othello stabs Iago but doesn't kill him and wants him to be tortured by the authorities and Cassio automatically assumed as governor of Cyprus. After being told Othello would serve time he decides he's done enough bad things to everyone and kills himself before giving one last kiss to Desdemona. For example in Act 5, Scene 2, Pg, "OTHELLO (to DESDEMONA) I kissed you before I killed you. Now, killing myself, I’m dying while I kiss you again. (OTHELLO kisses DESDEMONA and dies)."