Jealousy

             Thy Iago, oh honest Iago. How I should have known you were too good to be true. Jealousy was with you from the moment I gave Cassio Lieutenant instead of thy. Oh jealousy be thy cause of chaos. You planned to ruin all that was well. If only I would have known. I would have planned a way to keep order. "O beware, my lord, of jealousy: It is the green-eyed monster which doth mock the meat it feeds on" Ha! Tis not I who is jealous, it was you. The green-eyed monster you spoke of, was clearly yourself. My sweet, sweet Desdemona would still be alive if it were not for your jealousy and scheming. I let your lies take over me. I trusted what you were saying, and became jealous myself. Oh I am no better than thee. I got taken away from what was right and didn't realize it until everything was gone. Oh monstrous fate! 

Iago to Othello
Act 3 iii. 167-8

Literary Devices:
Symbolism - Green-eyed monster = Jealousy. Pictures, and gives a face to the green-eyed monster, which is within both Iago and Othello.
Abstract Language - Jealousy, which is a quality that cannot be perceived with the senses. Puts a name to what Iago wants Othello to be truly feeling, instead of leaving the reader to figure it out.
Begging the question - Iago argues that Othello should heed Desdemona and Cassio's relationship together and makes the assumption that he is already jealous. This causes and pushes Othello to really think about what may be going on, which is simply what Iago has been telling him.