The shedding of innocent blood:
(Guide to the Scriptures | MMurder.:Entry)
MURDER. See also Cain; Capital Punishment
To kill a person unlawfully and intentionally. Murder is a sin condemned from earliest times (Gen. 4:1–12; Moses 5:18–41).
Whoever sheds a man's blood shall have his blood shed by man, Gen. 9:6 (JST, Gen. 9:12–13; Ex. 21:12; Alma 34:12). Thou shalt not kill, Ex. 20:13 (Deut. 5:17; Matt. 5:21–22; Mosiah 13:21; D&C 59:6). Jesus said, thou shalt do no murder, Matt. 19:18. Murderers shall have their part in the second death, Rev. 21:8. Ye are murderers in your hearts, 1 Ne. 17:44. Wo to the murderer who kills deliberately, 2 Ne. 9:35. God has commanded that men should not murder, 2 Ne. 26:32. Murder is an abomination to the Lord, Alma 39:5–6. He that kills shall not have forgiveness, D&C 42:18. Any person who kills shall be delivered up to the laws of the land, D&C 42:79.
(Old Testament | Genesis9:6)
6 Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man.
(Old Testament | Exodus21:12)
12 He that smiteth a man, so that he die, shall be surely put to death.
(Old Testament | Exodus20:13)
13 Thou shalt not kill.
(New Testament | Matthew5:21)
21 Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:
(New Testament | Revelation21:8)
8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.
so... yes, second death exists... there had better be a good reason for those who end up there.
(Doctrine and Covenants | Section 42:18)
18 And now, behold, I speak unto the church. Thou shalt not kill; and he that kills shall cnot have forgiveness in this world, nor in the world to come.
so those in the church who know better, and still decide to kill anyways - we are responsible for acting on what we know. If you don't know any better? sure we make mistakes. But if you know beyond a shadow of a doubt, and still go around killing people? sorry charlie...