When Mercutio is slain, Tybalt realizes the gravity of the situation and the consequences of his actions. He understands that the conflict between the Capulets and the Montagues has escalated to a deadly level, and he may have a personal responsibility for the death of Mercutio. Additionally, Tybalt may realize that his own life is now in danger as he has stirred up more violence and hatred between the two families.
