Iago has some issues.

I'm not entirely sure as to whether pernicious little Iago is a lucky neophyte in his plans to destroy everyone in his way, or if he actually crafted and demised this plan to get back at Othello for giving Cassio a promotion over him. Needless to say, this dissident definitely has some personal problems to overcome. I'm not sure that I could have such a strong vendetta against someone over something as menial as that promotion. Let me see if I can re-piece the sequence of events that were Iago's plans. SO. Roderigo was in love with Desdemona, but Desdemona married Othello. Iago was angry at Othello because he gave a promotion to Cassio over him. Iago tells Roderigo that if Roderigo pays Iago, he will make it so that Roderigo will get Desdemona, which in turn will cause Othello to crumble. I'm not sure why Roderigo would agree to this subornation, but that's a different story. Despite Roderigo's attempts to sway Desdemona, he couldn't make it fly. But he was resilient. Iago then told Roderigo to start a fight with Cassio, because apparently Cassio was another contender for Desdemona, which resulted in Roderigo's death. With him gone, Iago played on Othello's jealousy to get him to think that Cassio and Desdemona were sleeping together. Iago had Desdemona's sacred handkerchief put into Cassio's hands. Then after Othello had a seizure, he was then transfixed over a conversation he heard which made him ignorantly positive that Cassio and Desdemona were in fact sleeping together. He then took matters into his own hand and killed Desdemona, which Iago's wife Emelia discovered ostensibly (whether she knew before or not the world will never know) and unfolded the truth that she was never cheating on Othello. She then realized that it must have been Iago the whole time and confronted him. Iago stabs Emelia, Othello stabs Iago, and Othello kills himself. Iago's jealousy turned out to be the foible that would prevent him from ever being Othello's successor. Poor Iago, although his plan was one large éclat.