Who is most at fault?
Romeo and Juliet
I believe that Friar Laurece is most at fault
Friar Laurence says, "A thing like death to chide away this shame, hat cop'st with death himself to scape from it; And, if thou darest, I'll give thee remedy" (5.1.74-76). This quote proves that friar gave the potion to Juliet to make her seem dead. Friar giving her this potion makes a lot of things in the story go wrong and he makes it worse instead of helping.
The nurse tells Juliet, "Hark ye, your Romeo will be here at night. I'll to him; he is hid at Laurence' cell" (3.2.140-141). This shows that Friar has been keeping Romeo hidden in his cell. Friar should not have been hiding Romeo and helping him and Juliet keep seeing each other. Friar knew that their families hated each other but continued to keep them together.
Romeo says, "Ah, dear Juliet, Why at thou yet so fair?" (5.3.101-102). Friar was supposed to give Romeo a letter telling him that Juliet was not actually dead, but Romeo never got it. Romeo came and saw Juliet looking dead and he then killed himself, which lead to Juliet also killing herself. Friar should have never given Juliet that potion.
Some people might argue that it was Juliet's fault but she would have never taken that potion if Friar had never gave it to her. Friar has been giving them ideas and plans on what to do to stay together even though there parents did not approve. The ideas he gave Romeo and Juliet all went wrong, they lead to bad things happening.