It seems that ultimately we cannot know for sure why Nephi killed Laban. Unfortunately, we do not have enough of the right details to come to a solid conclusion. We cannot prove Nephi's (or Lehi's) upbringing, their knowledge, and adherence to Jewish law, Laban's actual actions (what the actual threat level was), the secular traditions and treatment of death at the time were, and we cannot know Nephi's state of mind at the time of the killing. Ultimately, the question becomes unanswerable. All we can do is understand the conditions at the time and make guesses as to what may have caused Nephi to kill Laban.
Furthermore, we cannot know the type of interaction Nephi actually had with the spirit, what outside influences may have helped, or if he really, truly felt that he and his brothers (as well as the nations they would start) would be unsafe if Laban lived.
So what can we learn?
I think there are several great lessons that we can gain from a reading of these passages. Because the question is--in my mind--unanswerable, we need to look at the scripture differently. There are great things within its words that we can understand and take into our treatment of the gospel.
First, that we must interrogate the Spirit, or rather, we must be critical of what we learn and are taught, or even the answers we get from prayer. We should continually ask to receive witnesses to our questions, and repeated answers so that we may know the truth of all things. I feel that this shows that inquiry to God should consist of a running conversation. This is exemplified through the Book of Mormon as the brother of Jared shows us in coming up with solutions himself, after inquiring of God on multiple occasions. We too must pray fervently in order to not only receive an answer, but to comprehend the answer that is being given to us. As Nephi had a running dialogue with the spirit during this time, so should we in our prayers.
Second, we must doubt our actions and the actions we are told to take. We must ask why we are doing the things that we are doing. Why do we believe or behave the way we behave? In order to gain a true testimony, I understand it to be imperative that we question our beliefs and our actions.
Third, are clear-cut rules to be praised? When can they, or should they be altered or broken? This is an important question that everyone should ask themselves. I truly believe that God provides laws and commandments to us in our time and present culture. What Nephi said God commanded him to do cannot be judged off of our current cultural beliefs. We live in a very different time and within a very different culture. It is hard to place our moral understandings and beliefs on Nephi's actions. We can only analyze it from his possible perspective to gain an understanding of what could have taken place. This also means that we must constantly look at God's law verse reason or logic to determine the best course of action. Are God's laws the same for everyone in every instance? I do not believe that is the case. Humans are too complex to say simply that one law/commandment fits all. They are conditional, and often times fluid.
None of these things are detrimental to one's faith unless you allow it to become so. Rather, it is an opportunity to evaluate critically, and understand what questions are important and answerable. If the questions are unanswerable (as I feel this one is) how should the questions be treated? I believe the answer is to treat them (and all questions) critically, tentatively, and even suspiciously. However, we should also allow Nephi to be believed and given the benefit of the doubt in order to take what we can from passages of scripture that may, ultimately, be entirely unanswerable.