Narrative: Mormon certainly had a source for this story. We cannot tell how accurately Mormon is reproducing his source, because Mormon is crafting these events. In the official history, the interplay of Nephi’s prophecy and the murder of the chief judge may not have been as prominent as Mormon makes it here. The sequencing of events may have been different.
At this point, Mormon is dealing with the story, not Nephi’s role in the story. The actual events of the death of the chief judge would have been reconstructed after the apprehension of the murderer, but certainly a record of them would have been retained. Mormon is citing that record, of course knowing the culprit from the beginning. In the story we have the murder, and then the servants running to tell the people. This suggests that the murder occurred when the chief judge was not accompanied by a large throng of people, as the servants exit the location to tell others to come to witness.
This murder must be happening at the same time as Nephi is speaking to the assembled crowd, and so Mormon has the problem of trying to narrate sequentially what happened simultaneously. His choice is to emphasize Nephi’s prophetic role, and then to make this slight back-up in time to re-coordinate the essential events. That Mormon concentrates on Nephi and coordinates the murder to Nephi’s timetable further exemplifies Mormon’s editorial intent. It is Nephi and his prophecies that are important. This event is told because it substantiates Nephi as a prophet, not because Mormon is interested in the intricacies of the Gadianton government.
Mormon does tell us, in his own way, that this is a Gadianton murder. He notes that this is done under “a garb of secrecy.” This is Mormon’s literary clue to his readers that this murder should be seen in the context of the Gadianton murders for political power.