Iago became overwhelmed with jealousy when Cassio was chosen for the role of lieutenant, which he desired greatly, instead of himself. Out of anger, he admits his hatred for Othello and reveals his true intentions. He just wishes to use Othello to get to the top and then he will get rid of him as Othello will be of no use to Iago.

Iago speaks to himself about his desire to ruin Othello's life. He will use anyone in order to get to that point, even Desdemona. He wants to ruin their life because of his jealousy and anger. He doesn't want them to have a good life.

Iago confronts Othello about a probability that Desdemona was cheating on him with Cassio. He tells Othello that he should be careful because jealousy is a monster that thrives on ruining people's lives. It is ironic because Iago's jealousy led him to a plan to ruin Othello's life yet he is trying to warn Othello about how dangerous jealousy can be.

Iago speaks to himself gleefully almost about his plan. He knows that it will succeed because Othello's jealousy has caused him to change. His anger will cause him to do something irrational. Iago admits to himself that he uses Cassio and his kind and respectful personality to turn Othello's life over.

While talking to himself about who is better to kill, Roderigo or Cassio, he agrees that Cassio is better to kill because he will be the one who causes more problems for him. He views Cassio as a superior person because of his kind personality and his good looks that women often fall for. This causes another conflict as Iago becomes jealous of Cassio. If Cassio remains alive, everyone will continue to favour Cassio over Iago.

Iago gives reasons for his actions and behaviours towards Othello. He addresses his hatred and the fake appearance that he puts on to make sure no one suspects anything from him. He basically summarizes his whole plan in this one sentence. His plan is to cause Othello's downfall just like Othello damaged Iago's reputation and ego when he rejected his request for lieutenant.

Iago uses his cunning ways to convince Othello that Desdemona is very much able of cheating and deceiving people just like she did to her father. This seemed to convince Othello as her father also said the same thing in the first act. Desdemona's father warned Othello that she could do the same thing to him.

In order to convince Othello that Iago is on his side as his faithful and loyal friend, he admits that he tried to murder Cassio just for Othello. Othello is touched by the action as if it is a normal thing to do. He even says that Iago helps him figure how to act which tells us how much of Iago's manipulation has actually gotten into Othello's mind. It has gotten to the point that Othello listens to and approves of anything Iago says and does.

Iago convinces Roderigo to murder Cassio by making it look like it is for a good cause. He In reality Iago couldn't care less if Roderigo died or got caught because he wasn't as much use to Iago as he was before. 

Iago confronts Othello about his "suspicions" that Desdemona might be cheating on Othello with Cassio. He speaks about how he saw them on multiple occasions together. Yet how come Othello would just blindly trust whatever Iago was saying instead of asking his wife or even Emilia who is very close to Desdemona. He thought that Iago was telling the truth but in reality he wasn't.

Othello is never suspicious of what Iago does. Doesn't he question when Iago suddenly brings up a random topic regarding his wife and an affair behind his back? Of course, he has known Iago for a long time but he has also known both Cassio and Desdemona for a long time. Their characters and personalities are very respectful in nature and typically caring towards others. Why does he suddenly trust Iago over two of his closest friends?

Othello is seen as a very respectful and calm person. Everyone admires him for his work and his character. Yet we can see that his jealousy and doubtfulness towards Desdemona and Cassio has changed his character completely. He is now suspicious of everyone, violent and angry all the time. It is obvious that Othello has never behaved like this before when he shocks everyone by slapping Desdemona. They seemed surprised that Othello was even able to do such a thing.

Jealousy can cause anyone to go crazy and this applies to Othello as well. His jealousy caused him to go "mad" and completely changed him. He didn't seem like his past self at all after he visited Cyprus. Instead of controlling his jealousy and thinking about the situation properly, he lets the jealousy and anger get to his head and this ends up harming Cassio and killing three people (including himself).