He thinks Susanna is being unfaithful and swears revenge.
Later that night, Count Almaviva instructs Barbarina to return the pin to Susanna. However, Barbarina loses it and expresses her feelings through the aria “I have lost it, poor me.” Marcellina and Figaro hear about the situation. Figaro becomes overtly jealous after recognizing the Count’s pin, and further hearing that the pin is being delivered to Susanna. He thinks Susanna is being unfaithful and swears revenge. Marcellina fails to pacify him and belts out the aria “The billy-goat and the she-goat,” lamenting that where wild beasts of the opposite sex can get along, rational humans fail.