Nephi’s justification for killing Laban shifts from an examination of the legalistic aspects to the pragmatic implications. Nephi has been promised that he will be a ruler and a teacher over a people in a choice land. That choice land of promise has a condition attached to it. The people will prosper only as they follow Yahweh’s commandments. Nephi notes that “they could not keep the commandments of the lord according to the law of Moses, save they should have the law.” Perhaps Nephi could teach his children from memory, but for an entire nation to be governed by Yahweh’s laws, those laws had to be available. Those laws were on the plates of brass, and therefore the Lord had commanded that the brothers retrieve them and take them on the journey to the land of promise.
Nephi then reiterates the legalistic justification, that Laban had been delivered into his hands. He cut off Laban’s head.
There were any number of ways Nephi could have killed Laban. Even slitting his throat would be similarly bloody, but not as gruesome (at least to modern readers). Benjamin McGuire has suggested that Nephi also crafts this part of the story to parallel David and Goliath. They were parallel in that the less powerful were up against a much more powerful opponent. There were the implications for the future of Israel. There is the very obvious parallel in cutting off the head. There is probably an intended parallel in showing that, like David, Nephi was destined to become a king.