In this passage, Mormon underscores his real intent in describing wars. He acknowledges, but does not focus on, the casualties of physical war. The more serious casualties are of spiritual war. These wars are not simply battles for territory or tribute, but a war for human souls. Thus, when Mormon describes the dead, he does so in spiritual terms.
His two categories are those who listen to a good spirit and those who listen to a bad one. Each receives an eternal reward that is likewise cast in only two extremes: eternal happiness or eternal misery. While this fate surely applies to each individual as well, Mormon is using this categorization collectively. It may be directed more to the Amlicites than the Lamanites. Mormon has told this story because of the Amlicites, as witnessed by his editorial interpolation moralizing on the war’s events. That moral emphasizes the apostasy of the Amlicites and their acceptance of the curse on the Lamanites. For Mormon, these Nephites have become Lamanites. These who were with the Nephites are now opposed to them—and opposed spiritually, not just politically.
With that perspective, Mormon classes those who have died into the two categories of those who heeded a good spirit (Nephites) and those who did not (the Amlicites, and by extension, the Lamanites).