Both Mercutio and Tyblat were killed when fighting was not necessary. Tybalt wasnt happy that Romeo came to the Capulet ball, so he challenged Romeo to a fight. Romeo said no, but Mercutio was mad at Tybalt for the challenge, so he agreed to fight. Mercutio was killed in the fight, and then Romeo killed Tybalt. If Mercutio and Tyblat were not so prideful, all could have had a pleasant evening instead of fighting.