The real danger of the Zoramite separation from the Nephites had to do with their location, which Mormon makes certain his readers understand in verse 3. Antionum is a buffer zone between the Lamanites in the south wilderness and the interior of Nephite lands, specifically Jershon. Mormon expects his readers to remember that Jershon is where the Ammonites, the former Anti-Nephi-Lehies, have settled. These were people who were given the land for their protection, so that they wouldn’t need to take up arms again. A major line of defense was the land of Antionum, that was located between Jershon and the Lamanites.
With the defection of the Zoramites from the Nephite religion, the fear was that they would also defect from Nephite political influence and turn to the Lamanites. That would mean that there was a major hole in the Nephite defensive positions, and that the next land in line was one that was inhabited by a people who had taken an oath not to fight. It was therefore possible that opening Antionum to the Lamanites would allow a very deep incursion into Nephite territory. This was a very dangerous position.
When verse 4 speaks of the Zoramites entering into a correspondence with the Lamanites, the intent is not to describe an exchange of letters, but rather an alignment of religion and politics. That kind of correspondence would lead to the Zoramites becoming Lamanites, and therefore, enemies to the Nephites.
This story is interesting for its system of naming people. The first person named Zoram in Nephite history was a servant who accompanied Lehi’s family. Although we hear little of his tribe, it is associated with the Nephites. The name had a positive connotation, until now.
The second interesting name is Antionum, which so clearly includes the root antion, an intentional metonym. An antion was a measure of gold (Alma 11:19) and, therefore, a designation of a city that was concerned for personal wealth. Taken together, it would appear that Mormon is using these names to first tell his readers that the Zoramites in Antionum have rejected the Nephite principle of equality and looked to the elevation of wealth. That implication from the name alone will be borne out as we read about their practices.
The name Zoram is certainly not intended to be seen in a favorable light. What it might suggest is that these were Nephites who have now apostatized. That certainly fits the religious and political position of Antionum, but the transition of the name itself from basically good to a representation of the opposite of Nephite ideals may be Mormon’s way of reinforcing the lesson that apostate Nephites are the most dangerous of enemies.