Benvolio is above all a man of good will. He is a kinsman of Romeo, and no friend to the Capulets, but above all he wants to maintain the peace, and keep his friends Romeo and Mercutio from getting into scrapes. He also tries to save Romeo by giving an honest account to the Prince of what happened in the fight that left Tybalt and Mercutio dead. He is one of the more level-headed characters in the play.
Tybalt, on the other hand, is extremely hot-blooded. Like Benvolio, he is loyal, but in a different way. As a Capulet, he is so filled with hatred for the Montagues that he even wants to attack Romeo after Lord Capulet tells him not to. It is his aggressive and vindictive nature that leads to the fight in the streets, which led to his and Mercutio's death, and Romeo's banishment.