Taking a life is a complicated issue. I'm not going to address the many shades of grey (like mistakes (manslaughter) or war (sanctioned), etc.) and focus specifically on murder: the deliberate taking of another person's life for personal reasons or gratification.
To this end, we need to understand that there are three periods of spiritual growth, and the consequences of murder differ in each. These three periods are:
- Baptism to "spiritual enlightenment" (you'll understand the quotes when we get there).
- Post-"spiritual englightenment."
D&C 131:6 teaches, "It is impossible for a man to be saved in ignorance." The opposite is also true. 2 Nephi 9:25 teaches us:
...where there is no law given there is no punishment; and where there is no punishment there is no condemnation; and where there is no condemnation the mercies of the Holy One of Israel have claim upon them, because of the atonement.
The reality is, you can neither be saved nor punished in ignorance. These are important concepts. The Unpardonable Sin in context of this question is taking an innocent life — but that's from the perspective of understanding the gift of God. People who don't understand the gift of life, who have not been born again in the Holy Ghost, cannot be judged guilty of the Unpardonable Sin and therefore can be forgiven the sin of murder.
An example of this is the ancient apostle Paul who, as Saul, consented to the death of Stephen.
And I persecuted this way unto the death, binding and delivering into prisons both men and women. ... And when the blood of thy martyr Stephen was shed, I also was standing by, and consenting unto his death, and kept the raiment of them that slew him. (Acts 22:4, 20)
Saul did not yet understand the glory of the Lord and, through that glory, the amazing gift of life. He certainly understood "Thou shalt not kill," but by his day the Hebrews had effectively realigned the commandments to refer to the chosen people — and Stephen was no longer among them. He was ignorant of the depth and meaning of Jehovah's commandment and not a believer in Jesus. But because of his vision he became a believer and visited Ananias and was told:
And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord. (Acts 22:16)
Before baptism, restitution is repentance (including satisfying the laws of man, see Article of Faith 12) and baptism. Before baptism, a person may understand it's wrong to kill, but he or she doesn't understand it the way God understands it. They are ignorant.
A Bit of Background Before We Continue
But before we go into that in detail, let's briefly look at the Unpardonable Sin in general. D&C 76:31-37 tells us very specifically who those who succumb to the Unpardonable Sin are:
Thus saith the Lord concerning all those who know my power, and have been made partakers thereof, and suffered themselves through the power of the devil to be overcome, and to deny the truth and defy my power — They are they who are the sons of perdition, of whom I say that it had been better for them never to have been born; for they are vessels of wrath, doomed to suffer the wrath of God, with the devil and his angels in eternity; concerning whom I have said there is no forgiveness in this world nor in the world to come — Having denied the Holy Spirit after having received it, and having denied the Only Begotten Son of the Father, having crucified him unto themselves and put him to an open shame.
These are they who shall go away into the lake of fire and brimstone, with the devil and his angels — And the only ones on whom the second death shall have any power. (Emphasis mine)
Those who commit the Unpardonable Sin are not forgiven. They are never forgiven. They will be judged to reside in Outer Darkness (also called Perdition) for eternity. But note those conditions.
- One must know the power of Christ
- One must have been made a partaker of Christ's power
- One must then have allowed oneself to be overcome by Lucifer
- And denied the truth
- And denied Christ's power
The unpardonable sin isn't a mistake. It is not an error. It is a choice — an educated choice. A willing defiance of God's will.
Which is why it cannot be forgiven. This sheds new light on Matthew 12:32, which states that anyone who speaks against the Holy Ghost (in the context described above) will not be forgiven. Had Paul committed murder after his vision of Jesus, it would have been an unpardonable sin.
But is the Unpardonable Sin restricted to denying the Holy Ghost? If you think about it, one of the Holy Ghost's duties is to teach us all things realting to Christ and His gospel (John 14:26). Therfore, any choice to turn against the Lord is a denial of the Holy Ghost and therefore is unpardonable.
And that includes taking an innocent life.
** 3. Post-"Spiritual Enlightenment"**
Yes, this is out of order, but I just introduced it, so let's roll with it. Consider the following:
Rev. 21:8 But ... murderers ... shall have their part in the lake which burneth with fire and brimstone: which is the second death.
We've seen that phrase, "second death," before. Scroll back up and read D&C 76:31-37 again. Those who commit the Unpardonable Sin, who become sons (and daughters) of Perdition, and are cast out to Outer Darkness, they suffer the second death. (I explain more about the second death in this answer to another question.)
But, just to make it clear in our day...
D&C 42:18 And now, behold, I speak unto the church. Thou shalt not kill; and he that kills shall not have forgiveness in this world, nor in the world to come.
2. Baptism to "spiritual enlightenment"
However, what about the period of time between baptism and achieving the spiritual enlightenment descirbed in D&C 76? Alma 39:5-6 teaches us:
Know ye not, my son, that these things are an abomination in the sight of the Lord; yea, most abominable above all sins save it be the shedding of innocent blood or denying the Holy Ghost? For behold, if ye deny the Holy Ghost when it once has had place in you, and ye know that ye deny it, behold, this is a sin which is unpardonable; yea, and whosoever murdereth against the light and knowledge of God, it is not easy for him to obtain forgiveness; yea, I say unto you, my son, that it is not easy for him to obtain a forgiveness.
Alma's last statement, "it is not easy for him to obtain forgiveness" reflects the period of time between baptism and achieving the conditions described in D&C 76. It's hard ... very hard ... to receive forgiveness, because having already been baptised the options for restitution are, well... non-existent.
But, is there any scripture or teaching that suggests there is a payment to be made? No, there is not. How can you possibly return the potential and infinite value of a life? It's little wonder that it's hard, even when it's possible.
Before I sum up, I would like to clarify something. Another answer made a reference to a study manual that included a quote from Brother Ludlow. I visited LDS.org and ran through all the institute study manuals and couldn't find that citation. It may be an outdated manual. I have no doubt Bro. Ludlow said what he did, but he was not a prophet of the Lord. He's not wrong so long as you remember the difference between these two periods: between baptism and knowing the power of Christ, and thereafter. D&C 132:19,26-27 specifically teaches that entrance into the new and everlasting covenant (the Celestial Kingdom and marriage) can ONLY be had if no innocent blood has been shed. There's a lot to this that Bro. Ludlow's quote doesn't even touch, and so I would not take it at simple face value.
OK, that was lengthy, but to specifically answer your question: there are three periods to consider:
Before baptism, killing a person is forgivable due to ignorance of the law and the gifts of God through repentance (including satisfying the laws of man) and baptism.
After baptism, it's difficult, very difficult, to obtain forgiveness because, frankly, how confusing is "thou shalt not kill," really? On top of this, what restitution can be made as part of the repentance process? As I said, very difficult.
Finally, once you have met the expectations of spiritual growth (what I referred to for lack of a better phrase as "spiritual enlightenment") as described in D&C 76, having been born again, received the Holy Ghost, been made aware of Christ's power and partaken of it (a whole discussion unto itself), murder is 100% unforgivable.
Oh, by the way, there isn't a loophole here. If you think you can sneak a murder in by not accepting baptism, think again. If you knew to make such a plan, you knew at least something about what baptism means, and that means you fall somewhere between the middle and last periods. The Lord's not a fool. Cheers.