See also: Genesis 9:6; Exodus 21:12; Leviticus 24:17; D&C 42:19
The murderer, by terminating an individual’s earthly experience, sins grievously against the person he has killed. Those who murder steal the precious gift of mortal experience from another and set themselves in open opposition to God, the giver of life.
Further, murderers place themselves in a position where it is impossible to ask forgiveness of the one sinned against or to make restitution – at least in this life. So grievous is the act that the Prophet Joseph Smith said murderers “cannot be forgiven, until they have paid the last farthing.” (Teachings of the Prophet Joseph Smith, p. 189.) — Arthur R. Bassett, “Thou Shalt Not Kill,” Ensign, August 1994, pp. 27-28
“As to crimes for which no adequate restoration is possible, I have suggested . . . that perhaps the reason murder is an unforgivable sin is that, once having taken a life – whether that life be innocent or reprobate – the life-taker cannot restore it. He may give his own life as payment, but this does not wholly undo the injury done by his crime. He might support the widow and children; he might do many other noble things; but a life is gone and the restitution of it in full is impossible. Repentance in the ordinary sense seems futile.
“Murder is so treacherous and so far-reaching! Those who lose their possessions may be able to recover their wealth. Those defamed may still be able to prove themselves above reproach. Even the loss of chastity leaves the soul in mortality with opportunity to recover and repent and to make amends to some degree. But to take a life, whether someone else’s or one’s own, cuts off the victim’s experiences of mortality and thus his opportunity to repent, to keep God’s commandments in this earth life. It interferes with his potential of having ‘glory added upon [his head] for ever and ever.’ (Abraham 3:26)” (Miracle of Forgiveness, pp. 195–96) — Old Testament Student Manual, p. 291