Tybalt kills Mercutio. In Shakespeare's play "Romeo and Juliet," Mercutio is a close friend of Romeo. During a street brawl between the Montagues and the Capulets, Tybalt, a member of the Capulet family, challenges Romeo to a duel. Mercutio steps in to defend Romeo and ends up being fatally wounded by Tybalt. This event is a turning point in the play, as it leads to Romeo seeking revenge and ultimately results in the tragic deaths of both Tybalt and Mercutio.