This question has already been answered many times. Here is a comprehensive link for you: http://www.enotes.com/romeo-and-juliet/search?q=who+is+responsible+for+the+death
There are many characters who influence and aid the lovers in their untimely death, however in the opening prologue the blame is placed on the waring families. 'The fearful passage of their death-mark'd love, And the continuance of their parents' rage, Which, but their children's end, nought could remove' (Prologue, 9-11). This is saying that their loved was always doomed due to the hate that their families could not put aside.
It could also be argued that fate was the most responsible. In Elizabethan times, people believed in the power of fate and that their destinies we written in the stars, hence 'star-crossed lovers'. In the opening prologue Romeo and Juliet's future has already been decided, their fate already sealed before the play has begun. This is alluded to again in I.4.106-113 where Romeo muses on his own death at the hands of fate.
There are four persons who are responsible for the death of Romeo and Juliet.
. Mercutio curses Romeo and Juliet's fate when fights Tybalt and is kilt
.The feuding families made it hard for Romeo and Juliet to be together.
.The Friar who allowed Juliet to cheat death causing Romeo and her family to think that Juliet is dead, causing Romeo to kill himself.
. The lovers themselves ready to commit sucide because of love, honestly I would not.
The Friar in my opinion was most responsible because he allowed the whole ordeal to continue. He could've said no to marrying them, but i think he felt it was his responsibility to keep their young love going. his plan to get juliet out of marrying paris was also a bad idea b/c it is what got paris, romeo, and juliet dead. Also, the two family's big fight also played a key role, b/c if they werent fighting then R&J wouldnt of had to sneak their relationship around.
hope that helps :)