What sort of person is Iago as he appears in Act I of Othello?
From the start, Iago comes across as a person who hates and looks down on other people. He is angry at Othello for promoting Cassio over him, and he derides Cassio as incompetent. Iago compares Cassio to an unmarried woman ("a spinster"), thereby insinuating he is a "virgin" to real battle experience. In putting down Cassio, he also manages to cast contempt on women, as well as educated people who get their information from books. There seems to be nobody he doesn't want to spit on.
Iago shows his hatred of Othello as he alerts Brabantio to his daughter's elopement with him, saying,
Even now, now, very now, an old black ram
Is tupping your white ewe. Arise, arise!
Awake the snorting citizens with the bell . . .
Iago is not simply delivering bad news, he is trying to turn Brabantio against Othello by having him imagine Othello as an animal having sex with his pure daughter. Iago manages, too, to liken the "citizens" of Venice to animals by describing them as "snorting." It is not just what he says, but how he says it that shows him as full of anger and contempt.
However, we can't fault Iago for being blind to himself. He knows what he is. He tells Roderigo he is a fraud:
Heaven is my judge, not I for love and duty,But seeming so, for my peculiar end.
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.