Why is Tybalt responsible for Romeo and Juliet's deaths?

In many ways, it could be said that Juliet's hotheaded cousin Tybalt is responsible for Romeo and Juliet's deaths. In Act III, Scene 1, Tybalt challenges Romeo to a duel. Romeo refuses, but Mercutio accepts the challenge in an attempt to protect his friend's honor. Tybalt kills Mercutio, which goads Romeo into combat. Romeo kills Tybalt and is forced to flee the city. Later, we learn Romeo has been banished from Verona and will be killed if he is caught returning. 

Thus, Tybalt is essentially responsible for Romeo's exile, and Romeo's banishment sets up the elaborate death potion hoax that ultimately goes awry and results in the deaths of both Romeo and Juliet. As such, although Tybalt doesn't directly kill Romeo and Juliet, his decision to goad Romeo into a duel is indirectly responsible for the tragedy that unravels in the later acts of the play.