Act 4, Scene 1

I think Iago knows Othello's psyche very well. Othello thinks in absolutes. Either Desdemona is a pure saint meant to be worshiped or she is a whore: there is no in-between. Once Iago has convinced Othello of Desdemona's "infidelity", Othello does what he is expected to do: he reverts to primal possessive anger.