If you compare Othello in Act 1 to the Othello in Act 4, you will see quite a few changes in his character that are caused by Iago. The Othello in Act 1 is confident in himself and in his love for Desdemona. The Othello in Act 4 doubts himself and Desdemona. Iago has created insecurity with Othello, causing him to feel inferior to such a man as Cassio, who is well spoken, white, young, and handsome. Othello's love for Desdemona has turned to hatred, as he becomes almost raving mad, working himself into such a frenzy that he has a fit. His once eloquent speech has become incoherent, coarse, replete with the animal imagery that marks Iago's speech.
To sum up, three arguments that Iago has changed Othello for the worse might be (1) Iago has created insecurity within Othello, making him feel inferior because of his race and age, (2) Iago has caused Othello to turn his love for Desdemona into hatred, (3) Iago has caused Othello to lose his honor and noble stature.