There are many examples for you to choose from. You could argue that, in addition to the deaths of the two protagonists, the feud also leads to Mercutio's death, who is neither a Montague nor a Capulet, and thus a relative outsider. Tybalt is killed by Romeo as a result of the same confrontation; Romeo also kills Count Paris when he comes to mourn Juliet's "death." Last, it is possible that Friar Laurence will face punishment of some sort due to his involvement in the potion plot. Thus, many people, some of whom were not even related to the Montagues or Capulets, have their lives destroyed as a result of the feud.