Romeo's killing of Tybalt is the turning point (climax) of the play--the point when something happens that will turn the action toward either a happy ending or a tragic one. What actions does the killing set in motion, with possible tragic consequences?

The killing of Tybalt sets in motion the tragic consequences of this tragedy. There is no turning back for Romeo after Tybalt's death. Things can only get worse for our star-crossed lovers. I might argue this shift begins with the death of Mercutio, but the death of Tybalt holds the real consequences for both Romeo and Juliet. Romeo will have to pay dearly for Tybalt's death. The Prince has to issue some kind of justice.