Iago represents evil and cruelty for its own sake. He is deeply unpleasant and this is revealed to the audience in no uncertain terms in his numerous asides. He even acts as an advocate for Othello’s character, telling the audience that he is noble and in doing so, comes across as even more villainous, now that he is prepared to ruin Othello’s life despite his acknowledged goodness.
So yes, Iago is evil, we can see this, but the other characters in the scene cannot.
