The most obvious culprit is the long-standing feud between the Montagues and the Capulets; thus, ignorance, spite, and a lack of forgiveness lead to the deaths of Romeo and Juliet. However, we can also blame circumstance: if Friar Laurence's messenger had reached Romeo in time, he would have known that Juliet was merely asleep, and not dead in the chamber. Thus, luck and faulty communication are also responsible for the final outcome.
We could also argue that Tybalt is to blame for instigating the duel which leads to his and Mercutio's deaths. If he had not been so hot-headed and prone to violence, a better outcome may have been achieved. Romeo, too, would also have to shoulder the blame for slaying Tybalt (and Paris).
Last, we could argue that Friar Laurence's behavior—that is, secretly marrying the couple, and encouraging Juliet to fake her own death—was irresponsible, and that he could have thought of a safer way by which to reconcile the two families.