Iodovico is shocked when he witnessed Othello's behavior towards Desdemona. During Othello and Desdemona's conversation, he accused her of cheating on him with Cassius, which is something Iago told him. He also told him she would try to deny it happening. Othello grew mad at Desdemona because he didn't believe her, which is when he slapped her. Iodovico was present and couldn't believe his eyes - his master wasn't so violent in the past.