Why Did the Buddha Establish the Precept of Not Killing?

On one hand, the Buddha established precepts in order to reveal cause and effect, enabling disciples to follow the law of cause and effect, avoid the punishment of cause and effect, and prevent disciples from falling into the three evil destinies or transmigrating endlessly through birth and death within the six destinies. The fundamental purpose is to protect disciples from harm, and also to keep them from harming sentient beings, so that there are no obstructive conditions on the path of practice and they will not be interrupted in the three evil destinies, rendering all their efforts in vain. On the other hand, the Buddha established precepts in order to nurture the disciples' mind of loving-kindness, to cherish sentient beings and protect their lives. The mind of loving-kindness is originally the mind of the Buddhas and Bodhisattvas; when sentient beings have a mind of loving-kindness, they will draw near to all Buddhas and Bodhisattvas, and in the future they will become one among those Buddhas and Bodhisattvas. If the Buddha had not established precepts, the disciples would have no code of conduct, their minds would have nowhere to rest, and they would be at a loss as to what to do; meditative concentration (samādhi) and wisdom would be unable to arise, they could not attain liberation, and could not reach the tranquility of nirvāṇa.