The Converted Lamanites
Once the Lamanites were converted to the Gospel, they "never did fall away." (Alma 23:6) They were truly converted and understood their position in God's eyes. The greatest guilt they felt was for all the killing they had done. Because they were truly converted and understood what they needed to do to remain guilt-free, they knew that they must never kill another man again.
And so when the Amalekites and Amulonites began to stir up the Lamanites to replace the king, the Anti-Nephi-Lehies held a counsel in which they decided and were commanded by the king to not take up arms in the coming battle. (Alma 24:6)
The Lord had softened their hearts and convinced them of their sins and were forgiven of their wrong-doings. (Alma 24:8)
To be sure they would not take up arms to fight, the Anti-Nephi-Lehies covenanted with God that they would never again "use weapons for the shedding of man's blood." (Alma 24:18) To show their covenant, they buried all their weapons deep in the earth.
"And thus we see that, when these Lamanites were brought to believe and to know the truth, they were firm, and would suffer even unto death rather than commit sin." (Alma 24:19)
When the Lamanites and Amalekites and Amulonites came to battle, the Anti-Nephi-Lehies "went out to meet them, and prostrated themselves before them to the earth, and began to call on the name of the Lord; and thus they were in this attidute when the Lamanites began to fall upon them, and began to slay them with the sword." (Alma 24:21)
The true Lamanites were stung in their hearts and subsequently repented. (Alma 24:25)
One thousand and five Anti-Nephi-Lehies were killed. More than one thousand and five Lamanites were converted, however.
Most of the killing was done by the Amalekites and Amulonites. They did not feel the sting when they murdered these innocent men, women and children. They were past feeling and were hardened in their hearts. (Alma 24:30)
Tangent: The Mission
I can't help but put a plug in for a movie I first saw as an 11-year old. It was Christmas Eve … probably 1986 or 1987 … and we went to my Grandparent's home for the evening. The featured film we watched that night was The Mission with Robert DeNiro and Jeremy Irons.
Without going into much detail, the gist of the film centers around a land dispute between Portugal, Spain and the Jesuit Priests. The land dispute has slave-trading implications. At the end of the film, Portuguese forces begin to enforce the Treaty of Madrid. One missionary (DeNiro) decides to fight the invading forces while the other missionary (Irons) decides to peacefully submit … much like the Anti-Nephi-Lehies. The Portuguese forces are much like the Amalkites and Amulonites where they mercilessly kill many innocent people.
The subplot in the movie is that of DeNiro's character who murders his brother at the beginning of the movie. As part of his repentance, he builds a mission with Irons. When DeNiro decides to fight, Irons explains to him that he will lose fogiveness of his sins if he kills again ... again, the same reasoning King Anti-Nephi-Lehi states in Alma 24:12.