Nephi's justification is to place his family in the position of Israel and the Exodus. They are leaving under command of the Lord, and will require the Law in their new life and location. To overcome his hesitance to carry out the will of the Spirit, Nephi must understand why, and that "why" is linked with the future ability of his people to know the commandments. Those commandments are contained on the brass plates, and they are therefore necessary to Nephi.
Legal Analysis of Nephi's slaying of Laban: Fred Essig and H. Daniel Fuller have written a long essay entitled "Nephi's Slaying of Laban: a Legal Perspective" (FARMS 1982). The authors provide an exhaustive analysis of relevant Hebrew law during Nephi's time. The analysis takes the form of a brief for the assessment of a defense strategy for Nephi. What laws might apply?
The authors examine burglary and theft as two laws which might justify Nephi's actions. They find them both failing. Burglary does not apply as Nephi's home has not been burglarized. Theft might apply, but the punishment for theft was payment of monetary damages, and never invoked the death penalty (Essig and Fuller pp. 6-7). The authors provide some background on the interpretations of a passage in Exodus which might apply to Nephi:
Exodus 21:12 He that smiteth a man, so that he die, shall be surely put to death. 13 And if a man lie not in wait, but God deliver [him] into his hand; then I will appoint thee a place whither he shall flee.
"The attitude of the Rabbis on the involvement of God in Exodus 21:13 differs in some respects... Their comments are found in an Amoraic utterance that was preserved by Simeon ben Laqish. The Rabbis' thoughts are developed through a discourse that associates Exodus 21:13 with 1 Samuel 24:13. The teaching is simple. "God is absolutely just. He will deliver over to an unwitting homicide (and Exodus 21:13 killer) only a man deserving death... In other words, the man [killed] will be, say, a murderer who, owing to lack of witnesses, escaped his proper punishment" (Essig and Fuller p. 20).
Essig and Fuller also note, however, that the interpretation of the culpability of the killer in this case does carry sufficient guilt that they would be worthy of the banishment to a city of refuge which would correspond to this crime, determined to be manslaughter (Essig and Fuller p. 21).
Essig and Fuller conclude:
"Biblical and Rabbinic law provides numerous defenses to a charge of murder. Many of these, however, such as the justifications of burglary, theft, self-defense, and minority status, do not apply to Nephi's slaying of Laban. Additionally, defenses like duress and attempted murder were probably not part of the Jewish law when the slaying occurred in 600 B.C.
Probably Nephi's strongest defense would have been the procedural requirement of two eye-witnesses for a murder conviction, for no witnesses are mentioned in the scriptural account. There would still have been the possibility of false witnesses, however.
A substantive defense offering some hope of protection is that found in Exodus 21:13 to killers who do not lie in wait. The variety of interpretations of the passage, however, make its application in 600 B.C. uncertain.
An additional defense not formally recognized by scholars, but with some support in the scriptures, is that of obedience to a commandment of the Lord. Although Nephi plainly comes within its requirements, it probably would nat have been recognized in Jerusalem at the time of the slaying" (Essig and Fuller p. 40).
Theological analysis of Laban's slaying: The legal analysis of Laban's slaying deals with the relationship of the killer to the community. Were Nephi to have stood trial for his action, the laws of the community would apply, and he would need to be reconciled against those laws, for expulsion from the community, or re-acceptance into community. While we can speculate on the applicable law, it was never brought to bear, and Nephi and his family were effectively removed from the community in any case. Therefore the laws of community had no hold upon him.
Regardless of where Nephi went, however, he was always subject to the law of God, and always answerable to that law. Indeed, it is the relationship of Nephi's action to the greater moral law that is at greatest issue in this story. How is it that one who was to be a prophet of God should kill, apparently in violation of the Decalogue injunction against killing?
The answer to that question can only come from God's perspective. The first question which must be asked is whether that injunction is considered inviolate to God. The answer is clearly no, if for no other event than that cited by Nephi in his exhortation to his brethren. God allowed, or caused, the death of Pharaoh and his army when the waters covered them as Israel fled before the army of Egypt. The death of some for the benefit of others appears to have precedence in the actions of the Lord. Therefore we cannot deny that the whispering of the Spirit to Nephi was valid. The action of taking a life for a larger benefit is within the known and accepted actions of the Lord.
What of Nephi's position before the Lord? This is best understood by examining the nature of the injunction not to kill. For what reason was such a command given? Let us take, for the moment, the position of the person killed. Assume that the person was righteous and that the killing was grossly evil. What is the eternal effect on the person killed?
Modern revelation of the Plan of Salvation indicates that the person killed will not have any eternal consequences to the shortening of his life on this earth. The Lord is able to make up the unfairness of this life, and provide all of its benefits to those who die. The important issue is the heart of the person, not the manner of their death. The injunction against killing therefore does not have any direct correlation to the quality of resurrection/exaltation of the person who is killed.
Who the is effected? Why the killer, of course. The person who comes to the personal capability to take the life of another has undergone a transformation of soul which is contrary to the celestialization process. In the Sermon on the Mount Christ extends this injunction against killing to an injunction against anger with another. As with the other examples Christ gives on that occasion, the purpose is to hone in on the essential problem, not the surface law. The essential problem is the hate and anger which might lead one to take another's life. In the eternal sense, that is the damage, not the death of another, but the death of the Spirit within the one who becomes capable of killing.
It is in this context that Nephi's actions have their reconciliation with eternal principles. Nephi is clearly hesitant to kill, is clearly not proceeding with an intent to murder. Nephi kills Laban only under the direction of the Spirit. Since there is no anger in Nephi's heart, the spiritual damage of murder does not apply to him. Nephi stands with a clean and open heart before the Lord, and is not guilty of an action which would eternally damn him.