There are several reaons to view the Friar as "guilty." Juliet has essentially no other adult to turn to for sound advice in the play. Her father has already proven himself rash, her mother distant, & her nurse loving but incompetent. The only other adult she can turn to is the Friar. He is the principle adult who could have steered the young couple in a proper direction. At first, the Friar seems like someone who should be trusted. He tries to warn Romeo of the temporary state of infatuation: “These violent states have violent ends / And in their triumph die, like fire and powder, / Which as they kiss consume… / Therefore love moderately; long love doth so. Too swift arrives as tardy too slow” (2.6.9-11, 14-15). But his words carry no weight, because his actions undermine them. He marries the couple anyway, despite the fact that he knows he is violating parental wishes, as well as wedding two people who have continually demonstrated to him a blatant disregard of reason. Of course, the Friar’s final misdeed comes when he gives Juliet the vial that will make her appear to be dead. The vial itself is suspicious. Given the stories circulating during the Renaissance in about the evils of Catholicism in general and priests in particular, this mysterious, unexplained substance feels occult-ish. Even if one could somehow dismiss this uncomfortable element, it is impossible to ignore fact that Friar Laurence has been instrumental in bringing the crisis to boil.
I don't know that the Friar is the one who should be the most blamed, however, carrying on in that idea, my logic would be this:
Well for one, the Friar came up with the plan, and then did not ensure that his message to Romeo was received. If i was going to help fake someone's death, I would make sure that the person most important to the idea, knew what was going on. If the Friar had not given Juliet the sleeping potion, or made sure that Romeo received the message of the plan, both of them would be alive. As the Friar himself says, "These violent delights have violent ends,
And in their triumph die; like fire and powder" (act 2 scene vi). He should have known better.