Prince Escalus is correct when in the final scene he declares of Friar Laurence, "We still have known thee for a holy man," meaning that Friar Laurence has committed no sins and is ultimately not to blame for the tragic events (V.iii.281-82). As Prince Escalus also argues, it is ultimately Lords Capulet and Montague who are to blame for all the deaths in the play. However, Friar Laurence certainly made some imprudent decisions that, unfortunately, escalated matters.
Friar Laurence's first mistake was in agreeing to marry the couple so suddenly and in secret. However, Prince Escalus is correct in implying that his deed of marrying them was neither a sin nor unlawful. In this time period, under the Catholic Church, it was perfectly legal for two young people at certain ages to marry without the consent of their parents. A girl could legally marry without parental consent at the age of twelve, while a boy could marry without consent at the age of fourteen (Catholic Encyclopedia, "Civil Marriage"). Since we know Juliet is thirteen and Romeo is old enough to no longer be in school, we know that Friar Laurence could certainly marry them without their parents' knowledge or consent. Friar Laurence even had good intentions in marrying them. He was hoping the marriage would unite the two families and create peace. However, unanticipated issues got in the way of creating peace, such as Tybalt's anger at Romeo and the fact that Paris was also asking permission to marry Juliet, making his act of marriage a mistake. The text is very clear that he had doubts about conducting the marriage from the start. We especially see Friar Laurence's doubts in his opening lines of the wedding scene, "So smile the heavens upon this holy act / That after-hours with sorrow chide us not!" (II.vi.1-2). In other words, in these lines he is rightly worrying that God or fate will create some sort of calamity to show that this secret marriage was a mistake. Therefore, instead of agreeing to marry them in secret, it would have been far wiser for him to act on the couple's behalf by making their intentions publicly known and at least trying to gain parental consent. Even at least parental knowledge would have proven to be far better than nothing at all.
The second mistake of Friar Laurence's that escalates matters is the decision to fake Juliet's death. Friar Laurence made that decision, not only to save Juliet from committing the sin of polygamy, but also to save his own neck and not anger any of his parishioners through his involvement. However, again, it would have been far wiser to act on Juliet's behalf by confessing to Juliet's parents what had happened and why he had done it. The end result would have still been the same: She would probably have still been disowned by her father, and she would have been able to join Romeo in exile in Mantua. Plus, had Friar Laurence made a confession, then her life and Romeo's would have been spared.
Regardless of Friar Laurence's mistakes, we must not forget that had Lords Capulet and Montague not decided to rehash the grudge associated with their longstanding family feud, a family feud that had been put to rest prior to the present Lords Capulet and Montague's generation, then of course the hatred and violence that killed not only Romeo and Juliet, but Tybalt and Mercutio as well, would never have existed. Therefore, it is really Lords Capulet and Montague who are to blame for their own decision, rather than Friar Laurence.