Friar Lawrence in "Romeo and Juliet" is responsible for the deaths of Romeo and Juliet for these three reasons:
1. Friar Lawrence meddles in the feud between the Capulets and Montagues by marrying the two lovers. In doing so, he seeks to unite the families; however, dissension increases when in Act III Romeo enters and seeks to callm the tensions. He tells Tybalt,
I see thou know'st me not/....I do protest I never injured thee,/But love thee better than thou canst devise/Till thou shalt know the reason of my love (III,i,54-59)
Tybalt interprets Romeo gestures and words as mockery of him, "Thou shalt not excuse the injuries," and tells Romeo to fight. This is the first of the "peace-making" efforts initiated by Friar Lawrence that fail.
2. Of course, it is Friar Lawrence's idea that Juliet take the vial in Act IV in order to get her family to forget about Juliet's marriage to Paris. And, had he not married Juliet and Romeo, Juliet would not have been so desperate as to enter the cell of the friar and ask for help. Friar Lawrence tells her,
And this distilled liquor drink thou off,..../In the meantime, against thou shalt awake,/Shall Romeo by my letters know our drift,/And hither shall he come. (IV,ii,115)
3. However, Romeo unknowingly arrives at the grave of Juliet and believes her dead because Friar Lawrence fails to get his message to him that Juliet only sleeps. Rashly, Romeo kills himself; Juliet wakes to find him dead, and succeeds in killing herself because Friar Lawrence would not stay and prevent such an act. Instead, he uns off when he hears the guards coming, "Come, go, good Juliet, I dare no longer stay." His cowardice causes Juliet's death.