Friar Lawrence is well aware of the blood feud between the Montagues and Capulets, and so he is well aware of the dangers posed by a relationship between Romeo and Juliet. Yet he still agrees to enter into the plot to marry the two lovers, setting in motion a series of events that lead to their deaths. In his defense, however, it should be pointed out that he hopes that their union will bring about a rapprochement between the two feuding families:
For this alliance may so happy prove
To turn your households’ rancour to pure love.
Later, of course, it is Friar Lawrence, who the audience is aware is an expert in mixing medicines, who devises the sleeping potion that makes Juliet appear temporarily dead. Indeed, the Friar comes up with the whole plan, one which was obviously dangerous. So it could be argued that the Friar (and, one might add more persuasively, the Nurse) was responsible for the deaths of Romeo and Juliet. In his speech explaining the lovers' deaths to the Prince, he essentially offers (or seems to offer) to take responsibility:
...if aught in this
Miscarried by my fault, let my old life
Be sacrific'd, some hour before his time,
Unto the rigour of severest law.
But he also points out that the Nurse was privy to the wedding, and many have suggested that if a single character was to blame for the tragedy, it was her. In any case, the Prince assures the Friar that "we still have known thee for a holy man" and clearly does not seem to hold him responsible. It seems somewhat harsh to blame the the Friar, particularly as his plot to have Juliet feign her own death was a response to her threat to commit suicide before marrying Paris.