Summary: In Buddhism, “Killing” is defined by the technical mechanics of the mind, not by social laws. The unwholesome Kamma is complete only if Five Factors are present: Presence: A Living Being. Perception: Knowing it is alive. Intention: A mind bent on killing. Effort: Making the attack. Result: The being dies. If all five are met, the precept killing (pāṇātipāta) is broken. This rule applies equally to Self-Defense, Euthanasia, and Abortion—Kamma judges the intention to destroy life, not the justification for doing so.

Artistic digital recreation of Migalaṇḍika (an ascetic who looked like a monk) The Monastery Murder Massacre that happened in Vesālī during the time of The Buddha is not well known among lay people. It was because of this massacre that The Buddha made a rule about killing humans. For the first 20 years, there were no rules and this was the third rule that was ever created. Before this, the monks intuitively knew what was proper and there was no need for rules. As time went on, new problems arose and more rules were made. We have 227 rules that are recited every fortnight, but we actually have thousands of rules and variations of rules which were made over the time of the Buddha. The slightly abbreviated story below is mentioned in the first Pāḷi book of the Tipitaka translated from The Book of The Discipline (Vol 1), PTS, cc-by-nc:

A friend of mine once told me his story about how he had a knife in his hand and was raging with anger. He was going to kill his wife and her new found lover. Below is a short retelling of the story in third person. The Story My friend from the former Soviet Union did his first Goenka meditation retreat in 1999. The retreat was free and was very popular around the world. (dhamma.org). Before that, he was already into martial arts and Taoism and although they talked about meditation and controlling the mind, he did not really feel there was a solid technique to practice. After his first 10 day Goenka meditation course, he felt that he had found a good technique to control his mind. He did about three of these meditation courses since.