The answer to your question is based on your view of human nature. If a person believes that mankind's nature is basically evil, then he would argue that Iago is most definitely human and has just given himself over to his natural state.
If someone views man as flawed but with the potential to be good, then he might argue that Iago is inhuman because there does not seem to be one semblance of purity or goodness about him. Unlike many of Shakespeare's other villains, Iago does not even possess a logical or significant reason for causing the havoc that he does in so many of the characters' lives. In this sense he seems to be pure evil, much like the serpent from the Garden of Eden that he often resembles in Othello and Desdemona's Garden of Paradise.
Iago is one Shakespeare's 3 or 4 great characters: with Hamlet and Falstaff as his most engaging. Some call Iago a vice character, or pure evil. But, I think, he is certainly more human than these two archetypes. After all, he has more lines than Othello for a reason. We are drawn to his lies, because even when he is lying, he speaks true. He says things, mainly to Othello and Roderigo, that we all think but dare not say. That he uses these truths to manipulate and control is, dare I say, human.
Look at his speech to Roderigo, who he has just talked out of suicide:
Our bodies are our gardens, to the which
our wills are gardeners: so that if we will plant
nettles, or sow lettuce, set hyssop and weed up
thyme, supply it with one gender of herbs, or
distract it with many, either to have it sterile
with idleness, or manured with industry, why, the
power and corrigible authority of this lies in our
wills. If the balance of our lives had not one
scale of reason to poise another of sensuality, the
blood and baseness of our natures would conduct us
to most preposterous conclusions: but we have
reason to cool our raging motions, our carnal
stings, our unbitted lusts, whereof I take this that
you call love to be a sect or scion.
In this speech, he is not afraid to admit that love and lust are but flip sides of the same coin. This is a very humanistic view.
To keep Emilia as a wife for so long, to keep Roderigo as his "purse strings" for so long, to keep his enemy Othello so close to him for so long, to keep Cassio so trusting of him for so long, to keep we the audience so captivated for so long, Iago must reveal humanity. To appeal to humanity, one must show humanity. And Iago does so with language. He has the best lines in the play. His soliloquies are as engaging as Hamlet's.
Language shows man's humanity, and Iago is humanity's greatest liar.