Following Tybalt's death, the Prince and the royal families arrived at the crime scene. Benvolio told him that Romeo killed Tybalt after Tybalt had killed Mercutio. After hearing the story, the Prince banished Romeo from Verona and if he were to ever step foot in Verona again, he will be killed.
Who were the evil-doers who started this fight?
He explained to the prince that Tybalt was the one who started the fight and killed Mercutio and in turn, killed by Romeo. He said that Romeo didn't want to fight him and that the argument was silly.
And then Mercutio and Tybalt began to fight before. Romeo attempted to stop the fight before Mercutio was stabbed and Tybalt fled the scene before returning back to finish Romeo. They fought and then Tybalt ultimately was slain.