Othello is a play about a black 'noble moor' having the perfect marriage'. This perfect marriage, however, is destroyed by another mans deception and trickery. The mans motives are unclear but through planting the seeds of suspicion into the moors mind his marriage dies and in its place jealousy and anger grows to the point of death.
Iago starts off as a passive character in his series of suggestions that generally speaks in a polite manor as more of an acquaintance than a friend. Even early on he seems to present himself as a friend, in an attempt to gain Othello's trust, however. He does this by saying to Othello 'You were best go in'. This makes Othello think that Iago is a friend trying to help. This shows us that Iago plans develop early on to destroy Othello's marriage. When he has established the moors trust however he begins to make much more direct comments but are still subtle enough to make Othello think Iago is simply trying to make these comments as a concerned friend.
An example of this is when Iago says 'the cuckold lives in bliss'. Which simply makes Othello consider the possibility of Desdemona being unfaithful. Iago increases the intensity of accusations until he effectively is dictating the method of killing Desdemona.
'Do it not with poison, strangle her in her bed,
Even the bed she hath contaminated.'
Even at this point though he does not reveal his villainous intentions by suggestion himself to not kill Desdemona. This simply means that Iago is practically innocent, as he has not physically done anything. When Iago does this though he makes suggestions that plant images in Othello's mind that make him picture the situation from an angle he has never thought of before. An example of this...