Commandment number 6, “thou shalt not kill.”
The footnote to the LDS version expounds “kill” to mean murder. This is one of the most heavy, or hard to forgive sins. Alma instructs his son Corianton that the three most grievous and hard to forgive sins are to deny the Holy Ghost, to shed innocent blood, and sexual immorality. Murder is so hard to repent of for a number of reasons. First, part of repentance is to restore anything that was taken or removed, etc. Well, it’s impossible for anyone but Christ to restore life to a dead person. So it’s almost impossible to repent of murder. Likewise, with the other two, you can’t restore virtue and you can’t take back a denial of the Holy Ghost.
But what about situations where the Lord has commanded that people be killed? Does that not violate his own law? Some examples of this:
- Abraham commanded to kill/sacrifice Isaac – Genesis 22: 1-14
- Saul commanded to destroy and kill all of the Amalekites (men, women, children, live stock, everything) – 1 Samuel 15
- Nephi prompted by the Holy Ghost to kill Laban – 1 Nephi 4: 10-14
A part of this answer is found in a quote from Joseph Smith. “God said, ‘Thou shalt not kill;’ at another time He said, ‘Thou shalt utterly destroy.’ This is the principle on which the government of heaven is conducted – by revelation adapted to the circumstances in which the children of the kingdom are placed. Whatever God requires is right, no matter what it is, although we may not see the reason thereof till long after the events transpire” (Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith , 256.) Sounds like blind obedience. Well, when the one you are following is a perfect God, then such obedience is OK. But rarely is it blind with God. It seems he will always provide enough understanding, sometimes after the trial of our faith, though.
The commandment not to kill might be expanded to include all living animals. I’m not sure if plants are considered alive. Of course one meaning of living might define having a spirit as part of being alive. D&C 77: 2, is revelation to Joseph about Revelations 4: 6, wherein he says “that which is temporal in the likeness of that which is spiritual,” but limits that to man and beast, not plants. In any case, there is a difference between beasts and plants which makes it OK to kill and eat plants without any fear of breaking commandment 6 (see D&C 89: 14,17 – for the Word of Wisdom commandments that tell us specifically to eat plants and grains). Beasts should only be killed in times of winter, cold, famine and excess of hunger (see D&C 89: 12-15).
Also, in 3 Nephi 12: 21-26, Jesus says not only should we not kill, but we should also not be angry!
How much of the wrongs and the evils of the world are caused by anger? If we would follow the commandment to not kill, and the deeper meaning from Jesus to not even be angry, then that would probably solve 90% of the things wrong with humanity.