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.
He pretends to be loyal and dutiful to Othello to deceive and destroy him.
Iago, from the start, is a good person to flee from, as he shows himself to be hateful and lying—but he is a great villain!