I think they all played their part. The Nurse and Friar Lawrence kept the marriage a secret. The Nurse orchestrated it while the friar actually performed the ceremony. Neither told the appropriate authorities nor Romeo and Juliet's parents. Of course, they had their reasons. Lord Capulet certainly pressed Juliet into a corner. While he thought she was merely ungrateful, she was really already married. It's hard to hold him responsible though because Juliet could have told him the truth. Granted, she didn't want to be disowned, but she could have left Verona to live happily ever after with Romeo in Mantua. Lord Capulet pressed Juliet but it was her decision. The nurse can't be held fully responsible either because of her position as a servant. While she was like a mother to Juliet, she was still an employee. She wanted to help Juliet but she also couldn't really go against her wishes. Friar Lawrence is the only one, apart from Romeo and Juliet themselves, that had all the information and was free to act. Rather than taking the appropriate actions, Friar Lawrence seems to continue to try to fix things himself and keep the secret rather than telling the truth. He is the one who orchestrated the failed plan of pretending Juliet was dead. It is the disastrous downfall of this plan that leads to the tragic ending. Perhaps, we should hold the Friar most responsible.