In 1 Nephi chapter 4, Nephi and his brothers faithfully go back for the brass plates, which make up the five books of Moses. Following the Spirit, Nephi finds Laban, drunk and unconscious. The Spirit inspires Nephi to kill Laban.
The reason that Nephi had to kill Laban was because Laban had the plates of brass. His family needed these plates, or the scriptures, so they would not ‘dwindle in unbelief’. Also, he knew that his children, and his posterity, would also need the scriptures.
What he did not realize is that if he did not acquire the plates, we would not have the Book of Mormon, which is necessary to clear up the confusion resulting from pages missing from the Bible. We needed for Nephi to acquire the scriptures so that the Book of Mormon would exist.