Who was ultimately responsible for all of the death that occurred in the play? Why?

Ultimately, the two households (Capulet and Montague) are responsible for the deaths of Romeo and Juliet. The feud between the two families makes it impossible for Romeo and Juliet to be together. As Juliet says, "My only love, sprung from my only hate! Too early seen unknown, and known too late! Prodigious birth of love it is to me that I must love a loathed enemy"(61). 

If is was not for the two families fighting, none of the tragic events would have happened. There would not have been such detestation between the two families and Benvolio and Tybalt would not have been killed. Romeo and Juliet would not have had to hide their love and Romeo would never have killed Tybalt so he would never have been banished. If Romeo wasn't banished, Juliet would not have had to fake her own death. And if Juliet hadn't faked her own death Romeo would not have found her "dead" and neither Romeo or Juliet would have died. In the end, it all comes back to the hatred between the two families.