For forty years, I have written and taught in the BYU Law School about the technicalities of the laws of homicide in the Bible and in the ancient Near East. It is a complicated and technical legal subject. Here are a few brief comments that may be relevant to how the slaying of Laban may have been understood 2700 years ago.
When the Spirit quoted to Nephi words from Exodus 21:13–14, Nephi certainly would have recognized those words as coming from the Code of the Covenant in Exodus 21–23. Those words formed the basic requirements of the Law of Moses in Lehi’s day. Exodus 21:13–14 provided a three-pronged exception to the general law against homicide, if: (1) "a man lie not in wait," and (2) come not upon him "with guile," and (3) "the Lord delivers him into his hand" (Exodus 21:13–14). The expression, to be delivered into your hands, does not occur very often in the Old Testament, so it would have been rather distinctive. Obviously, Nephi had not been lying in wait, planning what to do. He had not been stalking Laban like a hunter who’s trying to kill an antelope. The details of the homicide law were further developed in Numbers 35, where hatred was a crucial element in finding someone guilty of homicide. Nephi would have been taught these words. He probably could recite these passages by memory. He had probably heard this text read at public gatherings, as required by Deuteronomy 31. His parents must teach their children the law, morning and evening.
In addition, a lot of homicide stories were found in the Hebrew traditions behind the Old Testament. Nephi would have known them, especially the story (found at the beginning of the book of Exodus) of Moses killing an Egyptian, that led to Moses fleeing into the wilderness in Midian for forty years. It will be Moses who almost uniquely in ancient law will provide an exception that differentiated an excusable accidental or unplanned slaying from a culpable presumptuous murder. These stories also were important and memorable. Interestingly, the Bible begins with a homicide, with Cain killing Abel, and the Book of Mormon begins with Nephi killing Laban. Both of these stories tell us something fundamental about God directing the affairs of what’s happening; on the one hand, God protects Cain when he has to be driven out, and on the other hand, that God values the scriptures enough that blood had to be shed in order that these scriptures could be taken on plates that would endure.
Ultimately, whatever the legal reasoning of the day, Nephi did what he did for one and only one reason, and that was because the Spirit constrained him and said, the Lord has delivered him into thy hands.
Welch, John W. "Legal Perspectives on the Slaying of Laban." Journal of Book of Mormon Studies 1, no. 1 (1992): 119–141.