Quoting from The Pāṭimokkha Rules
Translated & Explained
by Ṭhānissaro Bhikkhu
The non-offense clauses state that there is no offense for a bhikkhu who acts unintentionally, not knowing, or without aiming at death. In the Vinita-vatthu, unintentionally is used to describe cases in which a bhikkhu acts accidentally, such as dropping a poorly held stone, brick, or adze; removing a pestle from a shelf and accidentally knocking off another one. Not knowing is used in cases in which the bhikkhu deliberately does an action but without knowing that his action could cause death. An example would be giving food to a friend not knowing that it is poisoned. Not aiming at death is used in cases where the bhikkhu deliberately does an action but does not intend that action to result in death. Relevant examples include trying to help a bhikkhu who is choking on food by slapping him on the back and inadvertently causing his death; telling a bhikkhu to stand on a piece of scaffolding while helping with construction work, only to see the scaffolding collapse; describing the joys of heaven to an audience, only to have a member of the audience decide to commit suicide in hopes of going there.
Thus, to fulfill the factor of intention here, a bhikkhu must be acting intentionally, knowingly, and aiming at death.
The word-analysis covers all the same points—although it shuffles the terms around—when it defines intentionally as “having willed, having made the decision knowingly and consciously.” Without teasing out the differences in terminology, we may simply note the important point added in its analysis, which is that an act of manslaughter counts as intentional here only when the bhikkhu has made a clear decision to kill. Thus if he were to strike a person unthinkingly in a sudden fit of rage, without being clear about what his intention was, it would not qualify as “intentional” here. The Commentary seconds this point when it defines having made the decision as “having summoned up a reckless mind state, ‘crushing’ through the power of an attack.” The Sub-commentary does not explain crushing or attack here, but apparently they mean aggressively overcoming, through a brute act of will, any contrary or hesitant thoughts in the mind.
Quoting from Getting the Message (also by Ṭhānissaro Bhikkhu),
Killing is never skillful. Stealing, lying, and everything else in the first list are never skillful. When asked if there was anything whose killing he approved of, the Buddha answered that there was only one thing: anger. In no recorded instance did he approve of killing any living being at all. When one of his monks went to an executioner and told the man to kill his victims compassionately, with one blow, rather than torturing them, the Buddha expelled the monk from the Sangha, on the grounds that even the recommendation to kill compassionately is still a recommendation to kill — something he would never condone. If a monk was physically attacked, the Buddha allowed him to strike back in self-defense, but never with the intention to kill. As he told the monks,
"Even if bandits were to carve you up savagely, limb by limb, with a two-handled saw, he among you who let his heart get angered even at that would not be doing my bidding. Even then you should train yourselves: 'Our minds will be unaffected and we will say no evil words. We will remain sympathetic, with a mind of good will, and with no inner hate. We will keep pervading these people with an awareness imbued with good will and, beginning with them, we will keep pervading the all-encompassing world with an awareness imbued with good will — abundant, expansive, immeasurable, free from hostility, free from ill will.' That's how you should train yourselves."
-- MN 21