There are two sections to this instruction that is given to Helaman. The first is to note that there is a curse on the land, and the second that the Helaman is to keep the information about these secret oaths from his people so that they do not fall subject to the curse. Of course what we will see is that the people learn these secret oaths readily enough, and they do tend to cause destruction.
We may speculate on the nature of this curse and the reason for it. Alma suggests that there is a curse on the land. It has been there before, and it ties these ancient practices of the Jaredites to destruction. In the Mesoamerican context, it is quite probable that Alma understands that this “curse” is upon the land because there are plenty of other cultures around them that are aware of the secret oaths and murders. This information is likely present in the surrounding cultures, and when the Nephites learn of it and implement it, it will come from those other cultures, and not from the record of the Jaredites. Alma tells Helaman to keep this information private so as to make it more difficult. It will not make it impossible, however, because the memories of those tactics are already prevalent in the world about them.