Assigning blame for the deaths of Romeo and Juliet is a never-ending and ultimately futile endeavor.

Juliet's nurse enables Juliet in her plans to marry Romeo and be with him after he's banished from Verona. At the same time, she betrays her loyalty and love for Juliet in deference to Lord and Lady Capulet's demand that Juliet marry Paris. The Nurse pressures Juliet to abandon Romeo and marry Paris, which only intensifies Juliet's desire to be with Romeo. Blame the Nurse.

The Montagues and the Capulets are embroiled in a senseless feud which serves as the foundation and framework for all the action of the play, including the deaths of Romeo and Juliet. If anyone knows or even remembers the reason for the feud, it's never stated in the play. Blame the Montagues and the Capulets.

Lord Montague and Lord Capulet mindlessly perpetuate the feud, to the considerable detriment of both of their families. Blame Lord Montague and Lord Capulet.

The prince fails to enforce Verona's laws against fighting in the streets, which leads to the brawl between the Montagues and Capulets in the first scene of the play. He clearly lacks the authority or force of personality to enforce his further edict against brawling under pain of death, since two more deaths occur in the streets after his edict.

The prince himself says that he's to blame for the deaths of Romeo and Juliet for "winking" at the "discord" between the Montagues and Capulets. Blame the prince.

Lord and Lady Capulet insist that Juliet marry Paris against her will, which prompts Juliet to appeal to Friar Laurence for some way to avoid marrying Paris and to be reunited with Romeo after he's been banished. This leads to the sleeping potion and to the deaths of Romeo and Juliet. Blame Lord and Lady Capulet.

The most virulent and outspoken proponent of the feud is "fiery Tybalt," who is obsessed with warring with the Montagues and killing a Montague, any Montague, to assuage his own ego and restore the honor of the Montagues for some archaic reason.

If Tybalt hadn't challenged Romeo to a duel, Mercutio wouldn't have stood in for Romeo. Tybalt wouldn't have killed Mercutio, and Romeo wouldn't have killed Tybalt and been banished. With that, none of the business about Juliet's sleeping potion and Romeo's poison would have occurred. Blame Tybalt.

Mercutio is all too eager to fight with Tybalt, a man who Mercutio himself considered a deadly, first-rate swordsman. Predictably, Tybalt kills Mercutio; Romeo kills Tybalt; Romeo is banished, and so on. Blame Mercutio.

Romeo's cousin, friend, and confidant, Benvolio, is the person who suggests that Romeo attend the Capulet's feast uninvited. Romeo's appearance at the feast sets in motion the whole series of events that eventually leads to Romeo and Juliet's deaths. Blame Benvolio.

Paris pressures Lord Capulet into allowing him to marry Juliet, which leads to Lord Capulet demanding that Juliet marry Paris, which leads to the sleeping potion business, which leads to the deaths of Romeo and Juliet. Blame Paris.

Kindly Friar Laurence's well-meaning advice and elaborate schemes contribute directly to the deaths of Romeo and Juliet. He, too, enables Romeo and Juliet, marries them, and realizes too late that he's made a mess of things. Blame Friar Laurence.

Friar John fails to deliver the letter to Romeo in Mantua that was entrusted to him by Friar Laurence. While it's true that Friar Laurence fails to impress Friar John with the critical importance of the letter, Friar John is prevented from delivering the letter due to concerns about the plague possibly being in the house where an unnamed friar was living, who Friar John went to ask if he would accompany him to Mantua—a plague that is mentioned nowhere else in the play. Friar John made no effort to have the letter delivered to Romeo. Blame Friar John.

As an aside, the plague might have been foreshadowed, if not actually been brought down on that house in Verona, by Mercutio's "A plague on both your houses!" which he invokes several times before he dies from the fatal wound inflicted by Tybalt. Mercutio can be blamed for that as well.

The Apothecary contributed to the deaths of Romeo and Juliet by selling Romeo a vial of poison, which, as the Apothecary himself acknowledged, was clearly in violation of the laws of Mantua. If the Apothecary had obeyed the law, Romeo would not have killed himself thinking Juliet was dead, and Juliet would not have killed herself seeing Romeo lying dead beside her. Blame the Apothecary.

Peter, who couldn't read, and Abram, Balthasar, Gregory, and Sampson, who started the brawl in the first scene of the play, all played a part in Romeo and Juliet's deaths. Blame them.

As for Romeo and Juliet themselves, had they not fallen in love and impulsively gotten married, none of this unfortunate business would have happened, and Shakespeare's play would be a comedy called Romeo and Rosaline. Blame Romeo and Juliet.

No one individual is responsible for the deaths of the star-cross'd lovers, but a number of authority figures do nonetheless contribute in their own way to the final, tragic outcome. One such figure is the Nurse, who singularly fails in her duty to take care of Juliet. She knows that Juliet's family doesn't want her to go anywhere near a Montague. And yet the Nurse actively encourages Romeo and Juliet's illicit love affair at every turn, instead of carrying out Lord Capulet's express wishes as she's supposed to. The consequences of the Nurse's willful disobedience are tragic indeed.

The Nurse is supposed to be a mature, authority figure, yet at times she acts like a teenager, irresponsibly arranging Juliet's marriage to Romeo and its subsequent consummation. And when Juliet's parents turn against her, instead of offering care and support, the Nurse simply tells Juliet that she should marry that "lovely gentleman" Paris. Juliet is only a naive young adult; her emotions are all over the place. What she needs is some wise, stable guidance from an adult authority figure. Sadly, she doesn't get it from the Nurse, whose dereliction of duty leaves Juliet all alone in the world. Forced back on her own limited resources, Juliet makes the fateful decision to drink Friar Lawrence's sleeping potion, a decision that eventually leads to the deaths of both her and the love of her life.

We might blame the Nurse, in part, for the deaths of Romeo and Juliet because she helps Juliet to deceive her parents.  She knows not only that the Capulets hate the Montagues, Romeo's family, but also that they intend for Juliet to marry Paris.  Rather than let Juliet's parents know about the attraction she witnesses between them at the party, she actually helps Juliet to send secret messages to Romeo concerning marriage.  Because of Juliet's youth and because the Nurse is actually in the employ of Juliet's parents, one might argue that she really ought to have felt more of an obligation to tell them the truth rather than conspire with Juliet, a very young girl, behind their backs.  

By the time the Nurse actually decides to do what her employers would want her to do (encourage Juliet to marry Paris) Juliet is already in too deep.  The Nurse ought to have known that Juliet would not relent in her marriage to Romeo, given how driven she was to make it take place.  Her judgment really seems pretty poor throughout the majority of the play.

The nurse is partly to blame for the deaths of Romeo and Juliet, because she encouraged and enabled Juliet to see Romeo.  

I would like to say that I don't think that the nurse is the only character that deserves blame.  Ultimately, Romeo and Juliet are most to blame, since they both committed suicide.  Friar Laurence also deserves a great deal of the blame.  As for the nurse, she encouraged Juliet to secretly meet with Romeo.

I must another way,
To fetch a ladder by the which your love
Must climb a bird's nest soon when its dark.

She covered for Juliet, and she sent letters to Romeo for Juliet.  I admit that the friar probably deserves more of the blame, especially since he performed the actual wedding; however, without the nurse's help, that wedding likely would not have occurred.  

Furthermore, the nurse knew about Juliet's love for Romeo, yet she began pushing Juliet to marry Paris.  This resulted in driving Juliet to seek more desperate measures to be with Romeo.  

Romeo is banished, and all the world to nothing
That he dares ne'er come back to challenge you,
Or, if he do, it needs must be by stealth. 
Then, since the case so stands as now it doth, 
I think it best you married with the County. 
O, he's a lovely gentleman!

I often think that had the nurse helped Juliet escape with Romeo to Mantua, they both might still be alive.