Here are some explanations about what it means to shed innocent blood:
Spencer Kimball wrote:As to the shedding of innocent blood, in one sense innocent blood might be thought of as the blood of those persons without guile, or of little ones who have not sinned. It might also be thought of as the blood of others whom the murderer deliberately kills. Surely the crucifixion of the perfect Son of God constituted the shedding of innocent blood. Joseph Smith's blood shed in Carthage Jail was innocent at least he said: "I am void of offense toward God and man." Modern scripture gives the following interpretation:
The blasphemy against the Holy Ghost, which shall not be forgiven in the world nor out of the world, is in that ye commit murder wherein ye shed innocent blood, and assent unto my death, after ye have received my new and everlasting covenant, saith the Lord God—. (D&C 132:27.)
President Joseph Fielding Smith gives us further light on this:
. . . Shedding innocent blood is spoken of in the scriptures as consenting to the death of Jesus Christ and putting him to shame. For those who have had the witness of the Holy Ghost, fighting with wicked hate against his authorized servants is the same, for if this is done to them, it is also done against him. For men who have had the light of the Holy Ghost to turn away and fight the truth with murderous hate, and those who are authorized to proclaim it, there is no forgiveness in this world, neither in the world to come.
...
A murderer, for instance, one that sheds innocent blood, cannot have forgiveness. David sought repentance at the hand of God carefully with tears, for the murder of Uriah; but he could only get it through hell: he got a promise that his soul should not be left in hell.
Although David was a king, he never did obtain the spirit and power of Elijah and the fullness of the Priesthood; and the Priesthood that he received, and the throne and kingdom of David is to be taken from him and given to another by the name of David in the last days, raised up out of his lineage.
To become perdition, Cain would have to commit the unpardonable sin. "No one can commit the unpardonable sin," Joseph Smith explained, ". . . until he receives the Holy Ghost." 11 Having received the Holy Ghost, one must then commit murder, wherein he sheds innocent blood. 12 Said the Prophet: "The unpardonable sin is to shed innocent blood, or be accessory thereto." 13 Innocent blood is the blood of the righteous. It is sanctified blood. And to shed innocent blood, one must come out in open opposition to the kingdom of God so as to assent unto the death of Christ. Explained Joseph Smith:
All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against Him. After a man has sinned against the Holy Ghost, there is no repentance for him [that is, he cannot repent, for he is as much darkened as he was once enlightened. See DHC, II, p. 23]. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it; and from that time he begins to be an enemy. This is the case with many apostates of the Church of Jesus Christ of Latter-day Saints.
When a man begins to be an open enemy to this work, he hunts me, he seeks to kill me, and never ceases to thirst for my blood. He gets the spirit of the devil—the same spirit that they had who crucified the Lord of Life—the same spirit that sins against the Holy Ghost. You cannot save such persons; you cannot bring them to repentance; they make open war, like the devil, and awful is the consequence. 14
The Prophet's implication that many apostates in his day may have become sons of perdition seems to repudiate the view that only a very few will become such during their mortal probation. The Lord, for example, spoke of the fourth generation after His appearance upon the Western hemisphere as being close to, if not directly in, this category of sin. 15 And in writing of this generation, Mormon said, "They did not dwindle in unbelief, but they did wilfully rebel against the Gospel of Christ." 16 This would seem to imply that many in that day may have become sons of perdition. Likewise, those who rebel against the Lord in the latter period of His millennial reign are pictured as coming out in open opposition against the truth. 17 Since they make open war with the Saints and seek to destroy them, it may be concluded that there will be many among them who will become sons of perdition.
Orson Pratt wrote:Are there any persons in this congregation who have murdered—who have shed innocent blood, and have done this in their ignorance of the law of God, or perhaps in the blindness of their minds, not knowing his law? There is a chance for you to repent. But if there is a person who has been enlightened by the Spirit of truth—a person who has received the gift of the Holy Ghost, and has put forth his hand to shed innocent blood, we do not call upon you to repent, for there is no repentance for you. It is a sin that you will have to meet before the bar of God. It is a sin for which there is no forgiveness in this world, nor in the world to come. It depends altogether how much light a murderer has before he commits the deed, as to his chance of forgiveness; but you have to suffer the penalty that is attached, which is death.
I am now preaching the first principles of the Gospel, and some of the most prominent sins of this generation I have named over before this congregation. Look abroad among the nations of the earth, and see the spirit of murder and bloodshed that exists in the hearts of millions towards their fellow-men. Look at the feeling of this generation in regard to our youthful Prophet, who was martyred for his testimony and for the revelations he received from heaven. In the year 1844 he was smitten down by the hands of his enemies. Even they are called upon to repent, if they did not know any better—if they did it in their ignorance. But if they did it with their eyes open, we would say to such persons, "There is no repentance for you." Perhaps, after they have suffered in the eternal worlds, there may be a possibility of some of those murderers who were not enlightened to find redemption at a certain period, and some degree of glory. This applies not only to those who put forth their hands to shed the blood of the servants of God, but to those who have sanctioned the dead; they are guilty also.
Are there any persons in this congregation who feel to sanction the shedding of the blood of innocent men in this generation? If you do, though you were not present to put forth your hand to stab the fatal blow, remember you are guilty. What said Jesus to the people in his day? "Wherefore, behold, I send unto you prophets, and wise men, and scribes; and some of them ye shall kill and crucify, and some of them shall ye scourge in your synagogues, and persecute them from city to city; that upon you may come all the righteous blood shed upon the earth, from the blood of righteous Abel unto the blood of Zacharias, son of Barachias, whom ye slew between the temple and the altar. Verily, I say unto you, all these things shall come upon this generation." How is it that those who lived two thousand years ago had to answer the blood of the martyrs slain five or six thousand years ago? Because they sanctioned the deed by putting to death the living Prophets sent to them. When a man puts forth his hand to shed the blood of a living servant of God who bears testimony of the truth, and has been sent by authority of God, that man is guilty of the blood of all the servants of God who have been slain from the creation; and all who sanction the deed will have to be punished with those murderers that actually perpetrated the same.