Balthasar plays a significant role in Romeo and Juliet's deaths. He is the character that informs Romeo of Juliet's death. When Romeo asks Balthasar how Juliet is, Balthasar states;
"Her body sleeps in Capel's monument,
And her immortal part with angels lives."
(Act 5, scene 1, lines 18-19)
By telling Romeo this, Balthasar inspires Romeo to kill himself. Therefore, Balthasar has led Romeo to killing himself.
Nurse also has a large role in bringing about the end of Romeo and Juliet. Nurse was the one who revealed Romeo's identity to Juliet, and then despite the fact that Romeo was a Montague, a sworn enemy of the Capulets, Juliet's family, Nurse proceeded to facilitate their meetings. Nurse is also the one to provide the rope ladder so that Romeo and Juliet may consummate their marriage of their wedding night.
"Now, nurse, what news? What hast thou there? The cords That Romeo bid thee fetch?
Ay, ay, the cords."
(Act 3, scene 2, lines 34-35)
Thus, Nurse makes it so that Romeo and Juliet would rather die than be separated from each other.
Friar Lawrence plays perhaps the most significant role in causing the deaths of both Romeo and Juliet. He is not only the one to marry them in secret, but he also devises the plan for Juliet to fake her death;
"Take thou this vial, being then in bed,
And this distilling liquor drink thou off;"
(Act4, scene 1, lines 93-94)...