Iago hates Othello because he’s jealous. He knows that Othello is more powerful, more respected, and more successful than he is, and this makes Iago feel inferior. He also resents the fact that Othello has married Desdemona, a woman who Iago would like to have for himself.
In addition to being jealous and resentful, Iago believes that Othello has wronged him in some way and therefore deserves to be punished. He believes that Othello is responsible for his own misfortunes and therefore deserves to suffer for them as well.
Finally, Iago is afraid of what will happen if he remains loyal to Othello instead of becoming an enemy of him: namely, that Othello will harm or destroy him for his disloyalty.
