The Lord will not allow this mission to fail. He will not allow Laman and Lemuel to kill Lehi and Nephi. He intervenes with Laman and Lemuel as little as possible but enough to suit His purposes. In this particular instance, they actually heard the voice of the Lord which did chasten them exceedingly. Of this event, 1 Nephi 17:46 says, he has spoken unto you like unto the voice of thunder, which did cause the earth to shake as if it were to divide asunder. It took the voice of God to control these rebels. Again, we are amazed that they see so many miracles, even hear the voice of the Lord, and yet they still will not hearken to the words of the Lord.