William Shakespeare's play Romeo and Juliet ends tragically with both of the young lovers dying. Assigning blame in the play is complicated, as we cannot construct some sort of scientific experiment to examine what would have happened if one element or another in the play had been different; in other words, with literary texts as with history, unlike science, we cannot actually examine counterfactual conditionals except as thought experiments.
The first crucial bit of evidence we have relevant to this question is the prologue, which describes the plot of the play as follows:
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life...
The use of the terms "fatal" and "star-crossed" suggest that the fates of the two young lovers were unavoidable, written in the stars, which give heavenly signs of earthly fate. Thus no matter what Friar Laurence and the Nurse did, the young lovers were fated to die.
At the end of the play, Friar Laurence seems to be admitting guilt. Although he meant for the best, in that he was trying to end the feud between the Montague and Capulet families, prevent the young lovers from committing the mortal sin of suicide, and help the young lovers out of sympathy, his actual actions in abetting them did lead to their deaths. He certainly feels guilty about their deaths, but had his letter not been misplaced, his plan would not have ended in disaster.
The case for the Nurse's guilt is less direct, but had she not encouraged Juliet in her romance with Romeo, the ending might have been happier. In the general moral context of the period, it was not appropriate for the Nurse to encourage a clandestine relationship.