Why is the nurse in Romeo and Juliet to blame for their deaths?
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.
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.