The first of the five precepts—Buddhist guidelines for an ethical life—is to refrain from taking life, often phrased as a vow not to kill living creatures. What qualifies as a living creature varies among Buddhist schools, but humans and all other animals are included. Broader interpretations count microorganisms and plants among living creatures. (Some Southeast Asian Buddhists, for instance, abstain from cutting down trees.) Violations of the first precept carry severe karmic consequences.
The first precept takes into account intent and motivation as well as actions, not unlike the Western legal system. Theravada Buddhism lists five factors that contribute to a violation of the first precept, including the desire to kill and whether or not the victim dies. So it is possible to violate the first precept without killing anything, and killing in self-defense or by accident is considered a lesser breach than premeditated murder. Because the precept allows for factors beyond the act of killing itself, the first precept can include not supporting killing in any way, including praising or encouraging the act in thought or speech.
From a Mahayana perspective, it is possible to kill without violating the first precept if the killing is of widespread benefit and the killer is motivated solely by compassion. A famous example of this utilitarian view in traditional texts is a story about the Buddha’s former life as a ship captain, found in the Upayakausalya, or Skill in Means sutra, in which he killed a murderous stowaway on his ship to save his crew of five hundred from being murdered or from blindly killing one another in an attempt to stop the stowaway. Though rare circumstances like this may not be considered violations of the precept, they still create karmic consequences. In modern times, there are cases of Vietnamese and, more recently, Tibetan Buddhist monks self-immolating to bring attention to their causes. Generally, however, Buddhist teachers err on the side of caution in endorsing anything that ends life, since the stakes are so high and pure motivation is deemed virtually impossible.
In the Buddhist teachings, there are no hard-and-fast rules connected to the first precept concerning vegetarianism, pest control, abortion, euthanasia, or capital punishment. Buddhists acknowledge that it is impossible to live without killing in some way. Thus Buddhist teachers typically advise practitioners to assess how their habits cause harm and reduce that harm as much as possible.