One of the most difficult steps of repentance is to feel that the Lord has forgiven you. Here we see the spirit of revelation in the words of Anti-Nephi-Lehi. After quite a struggle, they had received confirmation by the Spirit that God had taken away their sins. Apparently, it did not come easily, as they were the most lost of all mankind.
Having struggled so hard to obtain forgiveness from the Lord, they did not want to jeopardize their hard-fought innocence. The king says, the great God has had mercy on us, and made these things known unto us that we might not perish; yea, and he has made these things known unto us beforehand, because he loveth our souls. The content of verse 13 implies that if they had again turned to a violent lifestyle, that the Lord might not forgive them for their previous actions. This is according to the following principle, And now, verily I say unto you…unto that soul who sinneth shall the former sins return (DC 82:7). Hence the words of the king, perhaps, if we should stain our swords again they can no more be washed bright through the blood of the Son of our Great God.