the court goes, there is "without a doubt" evidence leading to Iago's guilt in being an accessory to the murder of Desdemona and an accessory to the suicide of Othello. He planted the roots of jealousy in Othello's mind, which led him to killing his wife, and eventually himself.
From the start of the story we see that Iago is upset with Othello because even though Iago has out-performed Cassio in battle, Othello promotes Cassio. This is seen in the beginning of the story when Iago is conversing with Roderigo (I, i, 28-31). You see that Iago is pissed when he asks Roderigo: "Now, sir, be judge yourself, / Weather I in any just term am affined / To love the Moor (I, i, 38-40)." Here, Iago is wondering why after what Othello has done, he should show any friendship to Othello (in Shakespearean plays and times, love had a contextual meaning of friendship). This is clearly the first piece of evidence to show Iago's discontent with Othello. Later in that scene you see Iago say "I am not what I am" (I, i, 65) which is his way of foreshadowing the false facade of an honest man he had given and will be giving to other characters throughout the play. .
One of the best pieces of evidence we have in this case is that of Iago's Act I, scene iii soliloquy. He states many things in lines 377 to 398, but there are a few we must investigate. First, he says "I hate the Moor" (380) which is clearly a motivation for assisting in his death. Second Iago mentions the whole "twixting sheets idea and how he feels Othello could be having some kind of affair with his wife. He says that it is merely a suspicion, but concludes that it will do. And last, we must look at how Iago knows that Othello is a very trustworthy man, and thus we see him later using this to his advantage, in convincing Othello to kill his wife.
More evidence of Iago's belief that Othello was having an affair with his wife can be seen when Iago says: .