Juliet's death might easily be blamed on Friar Lawrence's failure to communicate the intricacies of the plan to Romeo.
Friar Lawrence leaves Juliet after discovering Romeo has killed Paris and has taken his own life. Rather than stay in the tomb to console Juliet and to make sure that she doesn't do anything rash, Friar Lawrence runs away in fear.
Then, again, Juliet's death might easily be blamed on the foolish feud between the two families. Had the families put their differences behind them and moved on in a rational manner, the outcome may have been different. I think the families problems were the lesser cause of her death, but a cause of both Romeo and Juliet's deaths.
I really think the whole plan of faking her death, the failure of the communique' to Romeo, and the murder/suicide at the tomb all contributed to Juliet's death. When she awakened from the potion, she was alone in a tomb with her dead fiance and husband. She was overwrought and took her own life.
Had Juliet not been left alone in the tomb with the dead men, the outcome may have been different. But, then the play would not have been a tragedy. Since the idea of faking her own death was Friar Lawrence's, I think the responsibility of her death and the death of Romeo is with him.