This is a powerful argument and justifies taking the life of Laban. Also, there is no question that those who have a murderous heart as Laban did will eventually get their just reward. In this instance, the Lord wrought judgment on the neck of Laban earlier than He usually does—and He did it with the arm of Nephi.
Although this argument is powerful, one should be careful how it is used. Can it be transferred to different situations? Can the abused wife justify taking the life of her husband declaring, "It is better than one man should perish than that our family should dwindle and perish in unbelief"? Obviously not. This argument can be used for good or evil. For example, the exact same argument that the Spirit used to constrain Nephi to take Laban's life was used by Caiaphas to justify taking the life of Jesus Christ, Ye know nothing at all, Nor consider that it is expedient for us, that one man should die for the people and that the whole nation perish not (John 11:49-50). Thus, the key factor which justified Nephi was that he was expressly commanded of the Lord. Joseph Smith taught, "That which is wrong under any circumstance may be and often is, right under another…Whatever God requires is right, no matter what it is, although we may not see the reason thereof till long after the events transpire." (Teachings of the Prophet Joseph Smith, p. 255-256 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 17)
The following story shows how cultural traditions greatly affect how we view an incident like Nephi's killing of Laban:
"Over the years Hugh Nibley has enjoyed telling a story about his Arab students in the early 1950s who were required to take the basic Book of Mormon class at Brigham Young University. Knowing that the Laban episode had been troublesome to the moral sensitivities of many twentieth-century readers, Nibley was puzzled when these students found the story somewhat implausible but precisely for the opposite reason he had expected. Instead of being troubled that Nephi had killed the unconscious Laban, the students found it odd that he had hesitated so long. While the reaction of these Arab students cannot be taken as evidence of the attitudes of the inhabitants of the city of Jerusalem around 600 B.C., it does reinforce the point that different cultures have unique values and idiosyncratic legal expectations. Accordingly, modern readers should be willing to consider not only the implications and moral bearings of ancient scriptural events upon contemporary society, but also to approach these developments in terms of the ancient dispositions and legal norms that would have operated as guiding principles in the lives of people years ago." (John W. Welch, "Legal Perspectives on the Slaying of Laban," FARMS Journal of Book of Mormon Studies, vol. 1, no. 1 (Fall 1992), 140)