The Lord protects the free agency of his children and the rights of the righteous. Thus, when the wicked (either an individual or a nation) become so "steeped in iniquity" that they obstruct the righteous purposes of God, he removes them from the earth. Such was apparently the case at the time of the flood, and several times since then the Lord has commanded that the wicked should be destroyed. (See Deuteronomy 7:2; 9:4; Joshua 6:21.)
Laban had already attempted murder for his private gain (1 Nephi 3:13, 24-26) and was evidently a very wicked person; thus he apparently fully deserved the fate that befell him. Also, as we shall discover later in the Book of Mormon (Omni, verses 15-19), the brass plates did keep part of the Lehite nation from dwindling and perishing "in unbelief" for many hundreds of years, just as the Spirit had prophesied.