Literary: Mormon intentionally creates a parallel in these verses. In verse 11 we have many mourning for the dead, and also mourning because they fear for the eternal soul of the dead. These are contrasted with those who also mourn for their dead, but who have hope in the promises of the Lord.
It is tempting to suggest that the first category of people refers to the Lamanites, and the second to the Nephites, but this is probably not the case. Mormon worries very little about the Lamanites, and his concerns are almost uniquely Nephite. When Mormon discusses those who mourn, he is speaking of the loss of Nephite kindred. Among those Nephites, however, we have seen that there are differences of opinion about religion, with some believing the prophets, and some following the order of the Nehors. It is more likely that Mormon is referring to these internal divisions, and that it is Nephites who do not follow the gospel who “have reason to fear” (verse 11).