After Mercutio and Tybalt got into a civil brawl, which was meant for Romeo, Romeo comes in and tries to split the 2 men up. While holding Mercutio back Tybalt stabs Mercutio under Romeo's arms. Later, Romeo kills Tybalt out of pure rage from the death of Mercutio
While Juliet is waiting in the flower garden the nurse comes in crying about how someone has died. Once nurse can get her words together the cries about how Romeo killed Tybalt and that Romeo is banished Juliet starts crying.