Why did the Buddha establish the precept against killing?

The Buddha established the precepts, on the one hand, to reveal cause and effect, so that his disciples would follow cause and effect, avoid the punishment of cause and effect, and be prevented from falling into the three evil destinies, or from transmigrating ceaselessly through birth and death in the six destinies. The fundamental purpose was to protect his disciples from harm, and also to keep them from harming sentient beings, so that on the path of cultivation they would encounter no obstructive conditions, would not be interrupted in the three evil destinies, and would not have their efforts come to nothing. On the other hand, the Buddha established the precepts in order to nurture his disciples' compassionate mind, to cherish sentient beings and protect their lives. The compassionate mind is originally the mind of the Buddha and the mind of a bodhisattva; when sentient beings have a compassionate mind, they will draw near to all the Buddhas and bodhisattvas, and in the future they will become one among those Buddhas and bodhisattvas. If the Buddha had not established the precepts, his disciples would have no code of conduct, their minds would have nowhere to rest, 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 attain the tranquility of nirvāṇa.