Nurse says "Romeo can, Though heaven cannot. O Romeo, Romeo, Whoever would have thought it? Romeo!" (3.2.46) Juliet assumed something bad happened to Romeo so she tried to kill herself instead of trying to figure out on her own if something did happen to him.
Other people like Tybalt and Mercutio could have avoided all of the drama and violence and not have gotten themselves killed and not have been rude and viscous like Tybalt towards the Montagues.