I think that Coleridge's description of "motiveless malignancy" is quite appropriate in describing Iago in the first Act. His entire presence has been motivated by prior acts and the reader's introduction to Iago is one where he is already intent on taking down Othello. As opposed to conventional dramas where Iago's descent into evil would be gradual and one where redemptive characteristics would be present at the outset, the vision of Iago that is rendered is one where his evil intent is already present. The fact that Iago can be described as purely evil is apparent in how he seeks to use Roderigo as a part of his plan to make all others pay for Iago's own anger or hurt or whatever might be the root of his manipulation. When Iago says to Roderigo that "I am not what I am," it is a moment where the reader is forced to recognize that the character of Iago is complex and intensely driven to ensure that all others are pulled into his web of deceit and manipulation.