Romeo and Juliet, while old enough to fall in love and get married secretly, are still only children. They did have the courage to defy their families secretly but not out in the open. They went to the Friar and the Nurse for help.
The Nurse is actively involved from Act 1 where the two meet at the ball and then pine for each other and for their misfortunes for having fallen in love with the enemy. She is the one who listens to Juliet's wimpering and the Nurse runs errands to Romeo and to the Friar on the children's behalf. All this running around in secret has caused a more desperate situation than necessary, when she should have spoken up. However, remembering that she is a hired helper rather than a noble woman or family member may have had something to do with her compliance. She acts for Juliet out of love for the girl.
The Friar also should have spoken up. He is responsible for marrying the two and then arranging for Juliet's supposed death so that she may escape with Romeo to parts unknown. His note goes astray so that Romeo hears only that his wife is dead. She wakes up only to learn that her husband has killed himself, and she quickly follows.
Had the Nurse and Friar confessed to the marriage, the children wouldn't both be dead. However, the feud probably wouldn't be resolved and the two would have been forced to get an anullment since their families would not have stood for secret marriages.
Any number of people can be held responsible for the deaths of Romeo and Juliet. Their parents can be held responsible for their stupid feud, which causes the two young people to go to such extremes to prove their love. Some blame goes to Friar Laurence for suggesting that Juliet take a potion to make it appear that she is dead. Ultimately, however, I think Romeo and Juliet take most of the blame themselves. If Juliet had not taken the potion, Romeo would not have killed himself, thinking she was dead; Juliet, likewise, would not then have killled herself when she saw Romeo's dead body. They might have found a more mature way of handling the situation, such as appealing to the Prince for help or just running off together.
I think it is mainly Friar Lawrence. Although all the charecters were in some way part of it, he has done the most.
He was the one who wedded them, even though he knew that Romeo fell in love easily and just the day before was in love with Rozaline. He was the one who thought of the plan to give Juliet the potion to make her seem dead. The message to Romeo explaining Juliets "death" never got to him. He could have tried harder to get it to him, he could have sent someone out to go and find Romeo to give it to him in person.
After Romeo's death, when Juliet wakes up, he said he will leave her with the nuns. This is not taking responsibility for his actions and just dumping her in the easiest plce he could find.
so yea =]