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:
At one time the Buddha, the Master, was staying at Vesālī in the hall with the peaked roof in the Great Wood. At that time the Master talked in many ways to the monks on the subject of unattractiveness, he spoke in praise of unattractiveness, he spoke in praise of developing (the perception of) unattractiveness, he spoke thus and thus he spoke in many ways in praise of the attainment of unattractiveness. Then the Master addressed the monks:
“Monks, I wish to go into solitary retreat for half a month. No one is to approach me except the one who brings me almsfood.”
“Yes, Master,” the monks replied, and accordingly no one approached the Master except the one to take him almsfood.
Then the monks thought, “The Master has talked in many ways on the subject of unattractiveness,” and they dwelt intent upon the practice of developing (the perception of) unattractiveness in its many different aspects. As a consequence they became troubled by their own bodies, ashamed of them, loathing them. Just as a young woman or man, fond of adornments and with head washed, would be ashamed, humiliated and disgusted if the carcass of a snake, a dog or a man were hung around their neck, just so those monks were troubled by their own bodies, ashamed of them and loathed them. They took their own lives, took the lives of one another, and they approached Migalaṇḍika, a sham recluse, the recluse lookalike, and said, “Friend, please kill us. This bowl and robe will be yours.” Then Migalaṇḍika, hired for a bowl and robe, killed a number of monks.
He then took his blood-stained knife to the river Vaggumudā, and while he was washing it he became anxious and remorseful: “Indeed, itʼs a loss for me, itʼs no gain; indeed, itʼs badly gained by me, not well-gained. I have made much demerit because I have killed monks who were virtuous and of good conduct.”
Then a certain god of Māraʼs retinue, walking across the water, said to Migalaṇḍika, “Well done, superior man; it is a gain for you, it is well-gained. You have made much merit, because you bring those across who have not yet crossed.”
Then Migalaṇḍika thought, “So it seems it is a gain for me, that it is well-gained by me, and that I have made much merit by bringing those across who have not yet crossed.” He then went from dwelling to dwelling, from dormitory to dormitory, and said, “Who has not yet crossed? Whom do I bring across?” And those monks who were not free from desire became fearful and terrified, with their hair standing on end, but not so those who were free from desire. Then Migalaṇḍika killed a monk, on a single day he killed two monks, on a single day … three … four … five … ten … twenty … thirty … forty … fifty … on a single day he killed sixty monks.
At the end of that half-month, the Master arose from seclusion and addressed Venerable Ānanda: “Ānanda, why is the Sangha of monks so diminished?”
“It is because the Master talked to the monks in many ways on the subject of unattractiveness—he spoke in praise of unattractiveness, (.. as told before) Then, Master, hired for a bowl and robe, Migalaṇḍika killed a monk … on a single day he killed sixty monks. Master, please give another instruction for the Sangha of monks to be established in final knowledge.”
“Well then, Ānanda, call together in the assembly-hall all the monks that dwell near Vesālī.”
“Yes, Master,” he said. And when he had done so, he approached the Master and said, “Master, the Sangha of monks is assembled. Master, please do what you think is appropriate.”
Then the Master went to the assembly-hall, sat down on the prepared seat, and said:
“Monks, the samādhi by mindfulness of breathing, when developed and cultivated, is peaceful and sublime, an exalted state of happiness, and it stops and settles bad, unwholesome qualities on the spot, whenever they arise. Just as a big storm, when it arises out of season in the last month of the hot weather, stops and settles the dust and dirt in the atmosphere—even so the samādhi by mindfulness of breathing, when developed and cultivated, is peaceful and sublime, an exalted state of happiness, and it stops and settles bad, unwholesome qualities on the spot, whenever they arise. And how is the samādhi by mindfulness of breathing developed and cultivated in this way?
As to that, monks, a monk sits down in the wilderness, at the foot of a tree, or in an empty hut; he crosses his legs, straightens his body, and establishes mindfulness in front of him. ….(section on mindfulness of breathing removed)…..
Monks, when the samādhi by mindfulness of breathing is developed and cultivated in this way, it is peaceful and sublime, an exalted state of happiness, and it stops and settles bad, unwholesome qualities on the spot, whenever they arise.”
And in this connection the Master convened the Sangha of monks and questioned the monks:
“Monks, is it true that some monks have taken their own lives, have killed one another, and have said to Migalaṇḍika, ʻFriend, please kill us. This bowl and robe will be yoursʼ?”
“It is true, Master.”
The Buddha, the Master, rebuked them: “Monks, it is not suitable for these monks, it is not becoming, it is not proper, it is not worthy of a recluse, it is not allowable, it should not be done. How could those monks take their own lives … and say … ‘… This bowl and robe will be yours’? It will not give rise to confidence in those without it … And, monks, this training rule should be recited thus:
If a monk intentionally kills a human being or searches for someone to kill him, he too is expelled and not in communion.”
Thus the Master laid down this training rule for the monks.