Killing

From the surface, the precept against killing seems straightforward. One might even rhetorically ask, "Who in their right mind would kill?" However, this thinking often stems from the mistaken notion that this only applies to humans. The Buddha and the commentaries are clear in explaining that killing, or murder to be more precise, covers all sentient beings, not just humans. To quote the Dhammikasutta (Snp 2.14):

They’d not kill any creature, nor have them killed, nor grant permission for others to kill. They’ve laid aside violence towards all creatures frail or firm that there are in the world.

 Again in AN 3.163-3.164, where the Buddha specifies the qualities that lead to either heaven or hell:

“Someone with three qualities is placed in hell as if delivered there. What three? They themselves kill living creatures. They encourage others to kill living creatures. And they approve of killing living creatures.

Someone with these three qualities is placed in hell as if delivered there.

Someone with three qualities is placed in heaven as if delivered there. What three? They don’t themselves kill living creatures. They encourage others to not kill living creatures. And they approve of not killing living creatures. 

Someone with these three qualities is placed in heaven as if delivered there."

Clearly, the Buddha intended to mean that killing an ant or a mosquito is the same as that of killing a human (in terms of violating the precept, not in gravity). As the Buddha said, householders should lay aside violence towards all creatures frail or firm.

This is what makes this precept hard. While it is easy, if not almost automatic, to not just kill but also harm other humans, it is not so easy to extend the same compassion and restraint with animals, especially insects.

This precept is also not just about us doing the act ourselves. It extends to having other creatures killed and granting permission to others to do so. We must recognize that we are guilty and, in fact, liable for violating the precepts too when we get or allow others to kill. Additionally, as mentioned in the Dhammikasutta, we should also renounce violence altogether. When we approach things from the perspective of violence being permissible so long a we don't kill anyone, we are setting ourselves up for failure. Just because you broke a person's leg or destroyed someone's arm rather than murdering them does not make you any less liable for your actions nor render you more virtuous.

To give a definition to the precept, Ācariya Buddhaghosa gives the following in the Papañcasūdanī:

But as regards the particular terms, the phrase killing living beings means the slaughter of a living being, the destruction of a living being. And here a living being (pana) is, according to ordinary usage, a being (satta); in the ultimate sense it is the life faculty. "Killing living beings" is the volition to kill on the part of one who is aware, in respect of a living being, that it is a living being, and which (volition), manifesting itself through one or the other of the doors of body and speech, initiates activity resulting in the cutting off of the life faculty.

Requisites

Coverage of the precept aside, what counts as killing? In the first paragraph, we mentioned murder as the more precise term. This comes from the commentarial tradition. In the Sumaṅgalavilāsinī, Ācariya Buddhaghosa mentions five requisites for an act to be considered as murder (i.e., fall under the precept against killing):

Tassa pañca sambhārā honti: pāṇo (living being), pāṇasaññitā (awareness that a living being is there), vadhakacittaṃ (intention to kill), upakkamo (action [of killing]), tena maraṇan ti (death). 

One cannot be held liable for violating the precept if there is no living being to begin with, if one had no awareness that there was a living being to be killed, if one had no intention to kill, if no action was taken to act upon the intent, nor if the victim did not die.

These have parallels in modern legal systems today, wherein murder has requisites, while everything else fall under their own specific category or classification.

Performance

How does one perform the act? As mentioned in the cited verse from the Dhammikasutta, having something or someone killed and giving permission to others to do so also fall under the precept. According to Ācariya Buddhaghosa, there are six ways to perform or realize the act:

Cha payogā: sāhatthiko (by one's hand), ānattiko (by command, order, or instigation), nissaggiko (by weapons), thāvaro (by stratagem or traps), vijjāmayo (by occult sciences or sorcery), iddhimayo (by supernatural means) ti.

"By one's hand" means doing it with your hands or using melee weapons, such as swords or daggers. "By command, order, or instigation" means having someone else do the act on one's behalf in some way or form. "By weapons" means using a weapon that 'discharges' or is 'thrown', such as shooting a gun or a bow and arrow or launching a projectile. "By stratagem or traps" means using a fixture or a fixed or static trap, such as digging a pitfall, setting a snare, or positioning a mechanical device. "By occult sciences or sorcery" means employing mantras, curses, or magical spells to cause an event to happen or to conjure a particular outcome. "By supernatural means" means using psychic powers, which one develops through practice. The last two are differentiated by their underlying mechanism—vijjā harnesses the power of various esoteric/occult mechanisms and systems, while iddhi comes as a result of having developed them either intentionally or incidentally.

Irreparable violations

There are five specific violations of the precept on abstaining from killing which the Buddha has labeled as atekiccha  or irreparable, irremediable, or incurable in the Parikuppasutta (AN 5.129):

Pañcime, bhikkhave, āpāyikā nerayikā parikuppā atekicchā. Katame pañca? Mātā jīvitā voropitā hoti (murdering one's mother), pitā jīvitā voropito hoti (murdering one's father), arahaṁ jīvitā voropito hoti (murdering an enlightened being), tathāgatassa duṭṭhena cittena lohitaṁ uppāditaṁ hoti (spilling the blood of a Realized One), saṅgho bhinno hoti (causing a schism in the Saṅgha). Ime kho, bhikkhave, pañca āpāyikā nerayikā parikuppā atekicchā (irreparable) ti.

These five are said to be the worst of violations of the precept on abstaining from killing.