Romeo does not want to fight Tybalt because he has just married Juliet, and even though he has to keep their marriage a secret, he now looks at Tybalt as his own family. Romeo ends up killing Tybalt after Tybalt kills Mercutio, Romeo's best friend. At the end of Act III, Scene 1, Romeo blames his love for Juliet, lamenting that he tried to make peace instead of killing Tybalt outright when Tybalt first insulted him.
"O sweet Juliet,
Thy beauty hath made me effeminate
And in my temper soft'ned valour's steel."
Tybalt's death results in Romeo's banishment. With Romeo fled to Mantua, Juliet is left to face her parents' concern. Believing her to be grief-stricken over Tybalt's death and in need of distraction, they arrange a hasty marriage to Paris—which Juliet then fakes her death to escape. On hearing the news, Romeo rushes to Juliet's tomb to kill himself. Juliet, waking to see Romeo dead, then kills herself. In short, by causing Romeo's banishment and separating the lovers, Tybalt's death precipitates the story's tragic conclusion.