This is an interesting statement. The guilt of the people of the Nephites could not be laid at the feet of either Mormon or Moroni. They are not actually guilty of the blood of their people, and therefore would not have needed to write the Book of Mormon for their personal expiation. They wrote not because of their guilt, but because they want to finish the job that they had been unable to do in life. They felt responsibility for teaching, even if the people did not listen. Through the Book of Mormon will come another chance for their people to be redeemed, and to justify the mission that Mormon and Moroni desired to fulfill in their own lifetime, but from which they were prevented by the unbelief of their people at that time.