These events are being written in the third person, which is probably Alma recording them in his personal record where Mormon found them and probably copied them, rather than rewriting them. Whether the text is due to Alma or Mormon, either one was more interested in the miraculous aspects of the story than the specific details. Thus, we have the indication that some of the people who had come were the very ones who had scattered the flocks at the waters of Sebus, as noted in verse 21.
That statement, combined with the fact that the brother of one of the slain was present, confirms that the intruders were not thieves, but were powerful men in their own right. They lived near the king, and were likely from a competing clan. Their identity was probably always known by both the king and the servants who had gone to the waters of Sebus, and because of their position and power, no one could move against them. Ammon, however, did not know. As an outsider, who was not under the king’s control (remembering that he had not married the king’s daughter, which would have put him under the king’s control), Ammon could disrupt the political game that they were playing. It is entirely possible that Lamoni sent Ammon to the waters of Sebus specifically to be that kind of a disruption.
The brother of a man who was slain attempted to kill Ammon, but he himself was stricken dead. The miracles continued, and for that one, there was a large audience to attest that Ammon could not be killed.