Nephi persuaded his reluctant older brothers to accompany him as far as the wall of Jerusalem and went into the city, alone at night. He said that he was "led by the Spirit", not knowing in advance what he would do. As he approached Laban's house, he found Laban himself, armed and armored, passed out drunken in the street. He drew Laban's magnificent steel sword, and as he did so, the Spirit of the Lord told Nephi to kill him. Nephi was reluctant to do so, since he had never killed anyone, but the Spirit repeated the command, informing him that the Lord had delivered Laban into his hands. Nephi considered the cause, that Laban was a wicked man and had robbed them, and a third time, the Spirit explained "Behold, the Lord slayeth the wicked to bring forth his righteous purposes. It is better that one man should perish than that a nation should dwindle and perish in unbelief". The purpose was not riches, but to preserve the religious heritage of the Jews for Lehi's entire future lineage. Nephi then did as he was instructed and killed Laban with his own sword. He then dressed himself in Laban's clothing and armor, and in this disguise went to Laban's treasury. There, he encountered Laban's servant who had the keys. Posing as Laban, he ordered the brass plates containing the record he wanted to be brought to be taken to his brothers outside the walls. The servant mistook Laban for his master until they got outside the walls, when his brothers, also supposing Nephi to be Laban, started to run. When Nephi identified himself, Laban's servant also started to run, but Nephi held him long enough to offer him his freedom if he would come with them. The servant, named Zoram, agreed to do so, and he returned with the brothers to Lehi's wilderness camp.
I might reference Exodus 21:13 as well as numerous others Deuteronomy, Joshua, Judges, and 1 and 2 kings, as evidence of that the concept of the Lord delivering enemies into the hands of the Israelites, (or vice versa) was accepted, as were commands to kill them if that should occur. This is a view rather foreign to modern Western culture where God is viewed as distant and not actively involved in determining the outcome of battles or conflicts.
I also note that these brass plates could have been lost and destroyed along with the rest of Jerusalem had they remained there.