From Nagarjuna's Treatise on the Great Perfection of Wisdom
(Dharmamitra Translation)

Question: People are able to use their strength to be victorious over others and standing together as a country they are able to demolish enemies. Sometimes the skins and meat hunted in the field provide great rescuing benefits. What is the value of causing one to refrain from killing beings?

Response: One gains a state wherein there is nothing one fears. One becomes peaceful, happy and free of dread. Because there has been no harm on my part towards others, others too harbor no harmfulness towards me. On account of this one is never frightened and has no fear. Although a man who likes to kill may rise to the highest position where he is a king among men, still, he is not at peace with himself. But if one is a man who upholds the precepts, he may travel by himself and roam about alone, having no fearsome difficulties.

Moreover, any being who possesses a life span does not enjoy encountering a person who takes pleasure in killing. If one dislikes killing, all beings happily rely on him. Again, when the life of a person who upholds the precepts is about to come to its end, his mind is peaceful, happy, devoid of doubts and free of regrets. And whether he is reborn in the heavens or among men, he always gains a long life span. This constitutes a cause and condition for achievement of the Way. When he finally achieves buddhahood, his life span during which he abides in the world is incalculable.

Additionally, a person who kills beings experiences all kinds of bitterness and pain in body and mind. A person who does not kill does not have these manifold difficulties. This constitutes a great benefit.

Furthermore, the practitioner reflects to himself, "I cherish my own life and am fond of this body. Others are like this as well. How are they any different from me? Therefore I should not kill any beings."

Moreover, if one is a killer of beings, he is denounced by good people and is hated by his enemies. Because he is responsible for taking the lives of others he is constantly afflicted with fearfulness and is detested by them. When he dies his mind is full of regrets and he is bound to fall into the hells or into the realm of animals. If he emerges from those realms his life span is bound to always be brief.

Then again, even if one were able to cause there to be no punishments in later lives, no denunciation by good people and no detestation by enemies, still one should not deliberately take another's life. Why? This is a thing which should not be done by those who are good. How much the more is this the case where in both worlds one encounters the resulting retribution arising from the baseness and evil of one's offenses.

Furthermore, killing constitutes the most serious of offenses. Why? When a person encounters a life-threatening situation, he will not be sparing of [even the most] valuable treasures. He takes simply being able to survive as what is primary. This is analogous to the case of the merchant who went to sea to gather jewels. When he had just about gotten back from the great sea his boat suddenly broke apart and the precious jewels were all lost. And yet, he was overjoyed and exultant, throwing up his hands and exclaiming, "I almost lost a great jewel!"

Everyone thought this strange and said, "You lost all your valuable possessions and escaped without even any clothes on your back. How can you joyfully exclaim, "I almost lost a great jewel!"

He replied by saying, "Among all the jewels, a person's life is foremost. It is for the sake of their lives that people seek wealth. It is not that they seek to live for the sake of wealth."

It is for this reason that the Buddha said that among the ten bad karmic actions, the offense of killing is listed first. It is also the first among the five precepts. Even if a person cultivates all sorts of merit, if he still fails to uphold the precept against taking life, there is nothing to be gained from it. Why? Although one may be born into a place of blessings and nobility while also being possessed of strength and the power of an aristocratic background, if he still does not have a long life span, who is able to experiences this bliss?

For these reasons one knows that among all of the offenses the offense of killing is the most serious and among all of the meritorious practices, refraining from killing is foremost. In the world preserving one's life is the primary concern. How do we know this? Everyone in the world would agree to undergo the physical cruelty of corporal punishment, including even beating and flogging in order to spare their lives.

Then again, if the thought to take on the precepts arises in a person whereby he thinks, "From this very day on I shall not kill any beings," in doing this he has already contributed a gift to an incalculable number of beings of something which they prize as valuable. The merit which he gains thereby is also incalculable.

According to what the Buddha said, there are five great gifts. What are the five? The first is not killing beings. This is the greatest gift. Not stealing, not engaging in sexual misconduct, not lying and not drinking intoxicants are the others which are the same in this respect.

Additionally, the merit of practicing the samaadhi of loving-kindness is incalculable. Water and fire do not harm one. Knives and military weapons do not injure one. No matter what the evil poison, one is unable to be poisoned by it. These are the sorts of things which one gains from the five great gifts.

. . .

Additionally, the practitioner thinks to himself, "All things possessed of life, including even the insects, cherish their own physical bodies. How could one, for the sake of clothing, drink and food or for the sake of one's own body, proceed to kill beings?

Moreover, the practitioner should study the Dharma of the great men. Among all of the great men, the Buddha is the greatest. Why? He has perfected every wisdom and has brought the ten powers to complete fulfillment. He is able to cross beings over to liberation and he constantly practices loving-kindness and pity. It was through upholding the precept against killing that he arrived at the achievement of buddhahood himself. He then also instructed his disciples to practice this loving-kindness and pity. Because the practitioner wishes to study the practices of the great men he too ought to refrain from killing.