When I think of the several people who may have been responsible for the lovers' deaths in Shakespeare's Romeo and Juliet, the first character that comes to mind is the hotheaded Tybalt. If he had behaved in a more civilized, mature manner, leaving Romeo alone (as Capulet told him to—several times—at the Capulet's party), he would never have gone looking for trouble. Instead, Tybalt takes it upon himself to be offended (although Capulet does not feel insulted) and avenge this insult by going after Romeo.
Patience perforce with wilful choler meeting
Makes my flesh tremble in their different greeting.
I will withdraw; but this intrusion shall,
Now seeming sweet, convert to bitt'rest gall. (I.v.94-97)
In turn, Mercutio (another hot-head) gets into a fight with Tybalt. When Romeo tries to break it up, Mercutio is killed; in a rage over the death of his dear friend, Romeo kills Tybalt. Had Mercutio not been so caught up with his sense of self-importance, he might have ignored Tybalt and walked away.
Because the Nurse is critical about Romeo's part in Tybalt's death and encourages Juliet to forget about him and marry Paris instead, Juliet chooses not to take the Nurse into her confidence when she and Friar Laurence stage her death. Therefore, the Nurse has no way of knowing that she is not dead. Had she been aware, the Nurse could have been waiting at the tomb for Juliet's return to consciousness, alerting Romeo to what had happened and saving both of the young lovers' lives. This could also be said for Friar Laurence, who should have been more responsible.
It would seem that the ignorance of the fathers in their age-old feud, the blind loyalty of those serving the families—or related to them—uncontrollable tempers, and the poor planning of Juliet's staged death all contributed to the young people's deaths, making almost everyone to blame.