Why does Iago choose Michael Cassio as an instrument of revenge against Othello?
Iago gives the reasons for Cassio being the ideal candidate for his machinations in the closing lines of Act 1, Scene 3:
Cassio's a proper man: let me see now:
To get his place and to plume up my will
In double knavery—How, how? Let's see:—
After some time, to abuse Othello's ear
That he is too familiar with his wife.
He hath a person and a smooth dispose
To be suspected, framed to make women false.
He believes that Cassio is a true gentleman with a good personality. This means that Iago will not face any opposition from him and he would therefore be easy to manipulate. Iago further mentions that Cassio is good-looking and charming, which makes it easy for him to make women cheat on their husbands.
The purpose of Iago's dastardly plot is clear in these lines. He wants to assume Cassio's place as Othello's lieutenant, an honor he was denied when the general appointed Michael Cassio. Iago states that he will lie to Othello about Cassio being "too familiar" with Desdemona. It will be easy for the general to believe his lies because of the reasons stated above.
Iago is intensely bitter about the fact that Michael Cassio was appointed by Othello as his second-in-command. He believes that he is entitled to that position for a variety of reasons. He has served Othello for a much longer period and has been at his side during many battles. Also, Michael is a Florentine and is, therefore, an insider. Furthermore, Michael is an inexperienced soldier who has no practical knowledge of combat. Iago claims that he has only book knowledge and is a mathematician, hardly the type to occupy such an esteemed position.
Iago feels humiliated that he should hold the lowly position of ancient. This means that he is, essentially, only a messenger with no real authority or power. Cassio is therefore the ideal target in Iago's malicious plot to destroy the general because Iago hates Cassio and the young lieutenant possesses all the qualities to make Othello believe that he is having intimate relations with his wife.
In essence, if Iago's plan succeeds, he will have killed two birds with one stone. He will have destroyed Cassio and probably have attained his position. He will also have ruined Othello's relationship with Desdemona. Iago despises Othello with a passion.
In the end, Iago's malice triumphs. He manipulates the naive Cassio into a situation that leads to his dismissal. The trusting young lieutenant follows Iago's advice to approach Desdemona for help. Iago uses these factors to lead the unsuspecting, insecure, and gullible Othello down a path which culminates in the most profound tragedy.
Prospero is right in that Cassio's nature makes him a perfect candidate for exploitation--he is attractive, not without vice, and willing to use Desdemona's influence on Othello for personal gain (or, more accurately, recovery). There is one other factor, however, which makes Cassio a particularly good instrument of revenge: Iago hates him.
Iago hates Cassio because he was promoted instead of him. He also hates Cassio because he is naturally more attractive (in every way) than him (Iago says that Cassio has a "daily beauty" about him that makes Iago look ugly). In using Cassio, he can only cause him to fall further out of favour with Othello. This will eventually lead to Cassio's utter downfall. If Cassio falls, Iago is certain that this will leave the way open for him to rise and take Cassio's place.
He tries to make Michael Cassio provoke Othello and make him jealous of Cassio so that he would kill two birds with one stone, which would be a sweet deal and a swift progress. His main reason for doing this was that he wanted to inflict harm on both his superior and a candidate vying for a position that was rightfully his. He was being selfish by doing that, risking everybody to take over the position