Why is Romeo banished from Verona?

On the day that Romeo and Juliet are married, , Benvolio and Mercutio meet a group of Capulets, led by Tybalt, Juliet's cousin. Tybalt and Mercutio begin fighting. Romeo and Benvolio try to part them but Tybalt kills Mercutio. Romeo is so angered he fights and kills Tybalt. Romeo is banished from Verona.