In Act 1, Iago uses Roderigo for his money. He knows that Roderigo can give him money in exchange for his schemes and friendship. Having seen the other person's weakness, he maximizes on it for his own devious ends. In addition, he hates Othello for picking Cassio as his lieutenant (I.i. 7-32), so that he promises himself that he will not let this unfair treatment go unnoticed. He plans to take his revenge by telling Othello that Cassio is sleeping with Desdemona (I.iii.369-370) and proves it through the aid of his wife Emilia and his other innocent "subjects".
In Act 2, Iago begins to suspect that Cassio has a hidden desire for Desdemona because he praises her and Iago finds it as a means to his advantage. Because he knows that Roderigo will do anything just to win Desdemona, he convinces him that if he kills Cassio then he will have Desdemona, and Iago will take Cassio's place as Othello's lieutenant. It appears that Iago has become so engrossed in his plans that his imaginations have been confused in his own mind to be reality. Having been motivated by the urgency of his desire for revenge, he boldly plans to tell Othello that Desdemona and Cassio are having an affair so that Othello will discharge Cassio and appoint him as replacement for Cassio as lieutenant (II.i.286).
Iago's schemes spare nobody who comes against his plans. He even discredits Cassio to Montano who has high regard of Cassio's abilities and character. So when Roderigo attacks Cassio, Montano is also wounded. Iago appears gentle to Roderigo as he comforts him after the fight, the latter being unaware that he is just being used to achieve Iago's personal and vested interests. To make matters worst, he uses his wife to let Desdemona and Cassio meet so that the meeting will appear to be a proof of Desdemona's infidelity to Othello (II.iii.310).
Act 3 shows that Iago uses and performs well as a good actor that Othello does not suspect his true intentions and feelings as he pretends shock and casually planting the seeds of doubt in Othello's mind about Desdemona and Cassio's "true" relationship. He mentions the use of Desdemona's handkerchief in wiping Cassio's brow. This seems to be the start of Iago's success in convincing Othello because he has made him so mad and jealous. This jealousy is the final spark that ruins Othello.
In Act 4, Iago continues to play on Othello's emotions by suggesting that Desdemona has indeed been unfaithful by sharing her bed with Cassio and using Othello's sentimental gift, the handkerchief lightly. He proves that Cassio is indeed boasting about his conquest of Desdemona by talking with Cassio about Bianca while Othello is watching afar unseen. Since Othello does not hear the conversation but only sees the expression on Cassio's face, he is all the more enraged, convinced and insulted.
Finally, in Act 5 Iago and Roderigo waits in a street to ambush Cassio. When he senses that Roderigo has become more inquisitive about the jewels he begins to worry. So he stabs Roderigo and eliminates the obstacles along his way. Later, he kills his own wife for revealing the truth about the handkerchief and then Iago escapes.
In the end, Iago does not triumph over good. He is proven the villain and he loses all that he has worked for. He even loses his own wife, his ambition and his soul.
How can a man like Iago be so driven by his emotions that he has done so much evil in order to achieve his wicked plan and yet not actually achieve it after all the damages he has