In act 3, scene 1, Tybalt insists on fighting Romeo, calling him a villain. Romeo, recently wed to Juliet secretly, doesn't want to fight Tybalt, because he is Juliet's cousin. He tries to explain his "love" for Tybalt, but this angers Mercutio, who declares that he will fight Tybalt if Romeo refuses. Tybalt stabs Mercutio, killing him. This creates a sudden shift in Romeo's emotions; Mercutio was his close friend.
Enraged, Romeo turns on Tybalt in a passionate act of revenge. Romeo insists that "either [Tybalt] or I, or both, must go with" Mercutio's soul. They fight, and Tybalt dies. Immediately, Romeo realizes the error of this act, calling himself "fortune's fool."
In this act of revenge, nothing is solved. In fact, Romeo has intensified the conflict between the Capulet and Montague families. While they have a long-standing mutual grudge, he had been hoping to find some sort of peace in order to move forward in his new marriage to Juliet. His revenge has made that impossible, cementing the hostility between the two families and even hurting Juliet, the person he loves most, by murdering her cousin.
This scene shows that revenge creates further conflict and leads to devastating consequences. As the scene ends, Lady Capulet calls for the death of Romeo, highlighting the reality that those who seek revenge will be forever ensnared in a cycle of violence.