Juliet's death's responsibility.

    Juliet's death was morally in the hands of the Capulet and Montague family. Due to their past issues and and hatred towards one another, it was initially their fault in the first place. If both of these families had put their problems aside and reconcile, Romeo and Juliet would've never died in the first place. Both of these families were aware that these star-crossed lovers were in love and lust for each other. However, they ignored all of the signs of true love, and chose to be selfish until they realized it was too late to go back and change their ways. 
   Juliet was too in love to think about the consequences if she were to drink the potion that was given to her from Friar Lawrence. Friar Lawrence on the other hand, was very idiotic. He couldn't come to realize that this plan wasn't the one to follow. Instead, he decided to go with his gut and forget about his common sense until it was too late. The consequences that Friar and Juliet had to face was the impact that hit Juliet once she woke up to see Romeo by her side, breathless. Hence, she decided to follow Romeo's footsteps and commit suicide also. She not only died a second time, but also made Friar Lawrence look like a fool that he truly was, due to his actions. 
Also, if these families were to unite and respect each other ahead of time, Juliet could have still been alive, but that is not the case. Instead, Juliet had to have her heart broken the hard way. This would have all been prevented if the two opposing families decided to compromise and allow their children to happily marry one another. That is why Juliet's death was caused by the idiotic actions of Friar Lawrence, followed along with the feud of the Capulet and the Montague family.