Helaman gives these wicked people another sign, but the people were not righteous enough to deserve another divine manifestation. The fact that Nephi gives them another sign is, from their perspective, a bad sign. Although his precise predictions exonerated himself, the sign also meant that God was angry enough at their wickedness and stubbornness that he was on the verge of destroying them. Their impending destruction is detailed in the next few chapters. Another example of this sort of dramatic demonstration of God's power, happened between Elijah and the priests of Baal.
In a contest of divine power, the God of Elijah was pitted against the pagan god of Baal. Each god was given the opportunity to bring fire from heaven to consume the offering and demonstrate their power. Of course, it was the offering of Elijah which was consumed, proving undeniably whose god was real (1 Kgs 18:17-40). Such a demonstration of divine power to the wicked was immediately followed by their destruction, 'And Elijah said unto them, Take the prophets of Baal; let not one of them escape. And they took them: and Elijah brought them down to the brook Kishon, and slew them there.' We know that it is a 'wicked and adulterous generation' which seeks for a sign, but when a 'wicked and adulterous generation' receives a sign, it means their destruction is imminent.