In a time of need Romeo and Juliet turn to Friar, seeing as they cannot turn to their own parents due to the bid between the two families. While Friar is cautious of how quickly Romeo has fallen for Juliet and abandoned Rosaline, he subsequently still weds Romeo and Juliet in hopes it would heal the conflict between the Montagues and the Capulets. Although he only had good intentions for the family, his decision is naive and he did not reflect on how it could inevitably play a role on Romeo and Juliet.
Although most would argue that Friar was the cause of the two youngsters deaths, Friar did not kill them with his own hands. Yes, Friar did hold a big part in the leading up to their deaths, seeing as it was his idea to put Juliet in the death