Not every question is a good question. Recently there was an entire article on an lds website addressing the apparently troubling issue of why Nephi was commanded to kill Laban.
To which, I feel like, a good response is "Have you READ the old testament?" Another better response might be "if God's own answer to Nephi about why he should kill Laban isn't good enough for you, then what can be done for you? If you get another answer you think is better, should you trust it more?"
But perhaps, to give a more meaningful answer, I will quote Brigham Young about the sword of Laban
I lived right in the country where the plates were found from which the Book of Mormon was translated, and I know a great many things pertaining to that country. I believe I will take the liberty to tell you of another circumstance that will be as marvelous as anything can be. This is an incident in the life of Oliver Cowdery, but he did not take the liberty of telling such things in meeting as I take. I tell these things to you, and I have a motive for doing so. I want to carry them to the ears of my brethren and sisters, and to the children also, that they may grow to an understanding of some things that seem to be entirely hidden from the human family. Oliver Cowdery went with the Prophet Joseph when he deposited these plates. Joseph did not translate all of the plates; there was a portion of them sealed, which you can learn from the Book of Doctrine and Covenants. When Joseph got the plates, the angel instructed him to carry them back to the hill Cumorah, which he did. Oliver says that when Joseph and Oliver went there, the hill opened, and they walked into a cave, in which there was a large and spacious room. He says he did not think, at the time, whether they had the light of the sun or artificial light; but that it was just as light as day. They laid the plates on a table; it was a large table that stood in the room. Under this table there was a pile of plates as much as two feet high, and there were altogether in this room more plates than probably many wagon loads; they were piled up in the corners and along the walls. The first time they went there the sword of Laban hung upon the wall; but when they went again it had been taken down and laid upon the table across the gold plates; it was unsheathed, and on it was written these words: “This sword will never be sheathed again until the kingdoms of this world become the kingdom of our God and his Christ.” I tell you this as coming not only from Oliver Cowdery, but others who were familiar with it, and who understood it just as well as we understand coming to this meeting, enjoying the day, and by and by we separate and go away, forgetting most of what is said, but remembering some things. (Brigham Young (Journal of Discourses "Trying to be Saints, etc..." 19:36)
The sword being unsheathed is a symbol of the destruction of the wicked "until the kingdoms of this world become the kingdom of our God and his Christ". So the sword of the Laban really is a symbol. but it is a symbol of the destruction of the wicked. It is a symbol of what it actually is. Destroying the wicked to bring about the Lord's righteous purposes.
So, in reality, my Brigham Young quote is just the Lord's answer given to Nephi at the time:
1 Nephi 4:12 ... Slay him, for the Lord hath delivered him into thy hands;
13 Behold the Lord slayeth the wicked to bring forth his righteous purposes. It is better that one man should perish than that a nation should dwindle and perish in unbelief.