In Romeo and Juliet, Mercutio is seen as the comic relief in some scenes. He is quick to make fun of Romeo for his love for Juliet. He also warns Romeo that nothing good can come from his relationship and that it will only cause more problems between the two families. Mercutio is aware that Romeo and Juliet being together will only add to the strife that is already there between the two families. He thinks no good will come of it. He calls Romeo the lover, and therefore does not think he is capable of fighting Tybalt.
Mercutio decides to fight Tybalt, because he doesn't think Romeo can fight him. Tybalt kills Mercutio. Romeo grieves for his friend. He has great sympathy for his lost friend and realizes that what Mercutio had told him was true. Romeo believes that no good can come out of his love for Juliet. He knows he has to fight and kill Tybalt.
"Alive, in tirumph! and Mercuti slain! Away to heaven, respective lenity, and fire-eyed fury be my conduct now! Now, Tybalt, take the villain back again, that late thou gavest me, for Mercuito's soul! is but a little way above our heads, staying for thine to keep him company: Either thou, or I, or both, must go with him."
Romeo feels nothing now but pure anger and hatred for Tybalt. He knows what it will mean if he kills Tybalt. This sets up the tragedy that plays out for the rest of the play. Romeo's actions in this one moment set up the sadness and tragedy that is about to follow.