small pieces in the larger picture: Roderigo is just a pawn and a purse, Emilia Iago's wife is a tool to obtain things, and Cassio plays his part without knowing it. Cassio happens to have a drinking problem that Iago uses to his advantage, which causes Cassio to be discredited from lieutenancy. There does not seem to be an obvious reason why Desdemona had to be involved in Iago's plan. There are other ways to discredit Cassio and have Othello die, but since Iago has such strong feelings against them, he went to the extra, more risky lengths to not only discredit Cassio and have Othello die, but to discredit and kill both of them, as well as Emilia, Desdemona and Roderigo. Luckily Cassio is at least saved.
             What drove Iago to those extra lengths to build this elaborate plan started with the rumors that Othello slept with Iago's wife. Iago uses that as an excuse to tell us why he
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