Sorry Romeo, we have all agreed that you have been banished from Verona
Tybalt, Juliet's cousin, was mad because he found out that Romeo went to the party. Tybalt found Mercutio and Tybalt challenged him to a fight. In the middle of the fight Romeo was trying to break up the fight and Tybalt stabbed Mercutio under Romeo's arm.
How will I go on without Romeo and Tybalt
Romeo wants revenge for Mercutio's death, so Tybalt takes the challenge and fights Romeo. Romeo said to Tybalt we both have to die or you are going to die. Romeo was so mad he couldn't hold back and he killed Juliet's cousin Tybalt.
Romeo killed Tybalt, who killed Mercutio. The prince, the Montague family and the Capulet family agree that Romeo will be banished from Verona and will not be able to come back. If he does come back, then he will be killed.
Now that Romeo has been banished from Verona, Juliet was very sad for two reasons. One is that her cousin Tybalt has been killed by her husband and now she can't see her husband ever again.