Who is to blame for Romeo and Juliet’s deaths?

The question of who is to blame for the young lovers' deaths in Shakespeare's tragedy Romeo and Juliet is a subjective one with many possible answers depending on one's interpretation of the play. Prince Escalus himself comes to no verdict in the final scene, instead parting with the open-ended proclamation: "some shall be pardoned and some punished."
One could argue that Romeo and Juliet are themselves the most directly responsible, as it is by their own hands that the newly-wedded pair takes their lives. However, this argument ignores the larger picture that has driven Romeo and Juliet to this point.
On the other end of the spectrum, one could zoom far out and argue that, on a societal level, it is the long-standing feud between their families that is to blame for their deaths, thus placing the blame on Lords Capulet and Montague as the heads of each respective house. If there had not been this barrier to Romeo and Juliet entering into a relationship, and had Lord Capulet not been pressuring Juliet to marry Paris, the ending of the play could have been easily avoided. Indeed, at the end of the play both men agree to put aside their longstanding rivalry to prevent what happened to their children from ever happening again.
The strongest argument, however, is that Friar Laurence is the most to blame. As Romeo's close confidant and frequent adviser to the titular star-crossed lovers, it is Friar Laurence's advice that guides the course of their relationship, and it is his plan that ultimately goes awry. He is the one who gives Juliet the potion to help her fake her own death. He is the one who tries and fails to get a message to exiled Romeo to tell him of this plan. He is also the only one who could have really prevented Romeo and Juliet from killing themselves, but he arrives at the tomb too late—just after Romeo has already killed himself—and then he leaves Juliet there alone, enabling her to stab herself. In the final scene of the play, he tells the whole story to the lovers' fathers and the Prince, saying,

"and, if aught in thisMiscarried by my fault, let my old lifeBe sacrificed, some hour before his time,Unto the rigour of severest law."

Here, he acknowledges himself that he may have had some hand in how events unfolded and offers himself up to the mercy of the Prince.