Iago has a hatred over Cassio, as he was given the promotion which he thought that he himself deserved “I know my price, I am worth no worse a place.” He knows he deserves the position over Cassio who is “Mere prattle without practice” and he despises Othello for giving it to Cassio “I have already chose my officer.” To get his retribution, he envelops Cassio in his vengeance plot so he can receive the title of lieutenant after his disgrace. In his plot for revenge Iago decides he will have to side with Othello if he wants his plan to follow through “I follow him to serve my turn upon him.” This is similar to the modern term “Keep your friends close and your enemies closer” Which means to lure your enemies in and keep them unsure of your intentions, therefore making them more vulnerable for attack or revenge against them. Iago’s plan of revenge was very successful, and by doing this Iago was able to gain the position of lieutenant. He was able to foresee Cassio's weaknesses, which was his low tolerance to alcohol, which led to him being demoted. Being a master of deception Iago was also able to see Othello's weaknesses and by using them he was able to manipulate him into giving him the position. This proves to a great extent that the idea of revenge is central to Othello.
Iago's revenge is also fueled by his suspicions towards Othello and Cassio. He suspects both of them of adultery with his wife “I do suspect the lusty Moor hath