"Now take a person in this congregation who has knowledge with regard to being saved in the kingdom of our God and Father and being exalted, the beauty and excellency of the eternities before him compared with the vain and foolish things of the world, and suppose that he is overtaken in a gross fault, that he has committed a sin that he knows will deprive him of that exaltation which he desires, and that he cannot attain to it without the shedding of his blood, and also knows that by having his blood shed he will atone for that sin, and be saved and exalted with the Gods, is there a man or woman in this house but what would say, "Shed my blood that I may be saved and exalted with the Gods?"
All mankind love themselves, and let these principles be known by an individual, and he would be glad to have his blood shed. That would be loving themselves, even unto an eternal exaltation. Will you love your brothers or sisters likewise, when they have committed a sin that cannot be atoned for without the shedding of their blood? Will you love that man or woman well enough to shed their blood? ... I could refer you to plenty of instances where men have been righteously slain, in order to atone for their sins. I have seen scores and hundreds of people for whom there would have been a chance (in the last resurrection there will be) if their lives had been taken and their blood spilled on the ground as a smoking incense to the Almighty, but who are now angels to the devil . . . I have known a great many men who left this church for whom there is no chance whatever for exaltation, but if their blood had been spilled, it would have been better for them, the wickedness and ignorance of the nations forbids this principle's being in full force, but the time will come when the law of God will been in full force.
This is loving our neighbor as ourselves; if he needs help, help him; and if he wants salvation and it is necessary to spill his blood on the earth in order that he may be saved, spill it. Any of you who understand the principles of eternity, if you have sinned a sin requiring the shedding of blood, except the sin unto death, would not be satisfied nor rest until your blood should be spilled, that you might gain that salvation you desire. That is the way to love mankind."
[Source: Sermon by Brigham Young, delivered in the Mormon Tabernacle, February 8, 1857, printed in the Deseret News, February 18, 1857; also reprinted in the Journal of Discourses, vol. 4, pp. 219-220, at Mormon Bookshelf: Blood Atonement, http://mormonbookshelf.com/wiki/Blood_Atonement]