Which character is most responsible for the untimely deaths of Romeo and Juliet?

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.