1 Nephi 3 – 4
Nephi’s strong response may have been the more sure because he had so recently received his assurance from the Lord of the validity of his father’s prophetic mission. I’m sure that Laman and Lemuel only agreed to try taking their riches to buy the plates because such a plan made sense to the carnal mind. When that failed I don’t think anything could have convinced them to do more than hide outside the walls of the city.
When Nephi encountered Laban he probably did not recognize at the time that it was the Spirit that first enticed him to kill Laban – he probably thought it was his own boyish imagination as he looked at the sword “I’ll bet I could kill Laban right here.” When paired with the further promptings in hindsight he realized that it was the Lord who so prompted him. Once he recognized that it was the will of the Lord he immediately did what was necessary to fulfill the command that he had been given.