Sure, other characters played parts in the death of Romeo and Juliet but I think that Mercutio, because he drew his sword first (p. 995), was the main cause. If Mercutio was not slain by Tybalt, Romeo wouldn't have challenged, and killed, Tybalt (p. 996.) Because Romeo killed Tybalt, he got banished from Verona by the prince (p. 999.) I believe that if Romeo hadn't been banished the two lovers would've had no reason to kill themselves, they could have continued meeting in secret. One might argue that it is Balthasar's fault for reaching
Romeo with the news of Juliet before Friar John did (p. 1034) but there wouldn't have been any need for that letter if Romeo hadn't been banished for killing Tybalt. In the end I think that Mercutio's act of drawing his sword to challenge Tybalt had some very important long term effects.