Nephi went forth in power. That very manifestation of power came into conflict with the rulers of the various tribes. Mormon tells us that it was “because he had greater power than they.” The combination of politics and religion would require that the rulers of the tribes also be the leaders of the religion of the tribal organization. Nephi’s powerful presence undermined their authority because Nephi was clearly able to operate in religious matters more powerfully than these rulers. Because there was such a tight connection between religion and politics, the undermining of religious authority was tantamount to the undermining of political authority, and therefore these rulers “were angry with him.”
Mormon suggests that they were angry because “it were not possible that they could disbelieve his words…” What Mormon is telling us is that Nephi was preaching powerfully and effectively. Not only are angels ministering to him, but he has the power over death, as indicated in the next verse. These rulers cannot disbelieve what they see.