Textual: This insertion by Mormon indicates that he has taken a detour from what he originally planned to talk about. Nevertheless, it is hard to understand precisely what the detour was, because Mormon returns to the subject of the detour. What was Mormon’s original intent?
If we were to remove verses 4-19 we would have a reasonably consistent narrative of the events moving directly from verse 3 to verse 20. It would appear that the insertion was triggered by verse 4’s introduction of the mark of the Amlicites. In verses 4-12 Mormon explains the mark, and discusses the cursing of the Lamanites from a Nephite perspective. Now in verse 13 he indicates that he has take a detour and must return to the story of the Amlicites. Apparently he considers the discussion of the Lamanites to be the side-step, for he immediately returns to the story of the Amlicite marking and how that fulfilled commandments.
From this information we can reconstruct Mormon’s intent. In his narrative, Mormon pauses to make a specific moral point. He wants to show that the Amlicites are both wicked in their rebellion from the Nephites, and that they are fulfilling a curse by marking their foreheads. This general practice is using history to frame moral stories is frequently used in Mormon’s editorial style. The aside from which Mormon must return in his narrative is the excursion into the discussion of the Lamanites themselves. While a description of the Lamanite cursing is certainly related to what Mormon wants to say, he does indicate that it is only tangential. His purpose is to show that the Amlicites fulfill prophecy, and are therefore still a tool of God even though they have rebelled against God.