Sentience, killing, & extremes?

The Buddha always said killing, no matter the circumstance, is always unskillful & leads to negative Rebirths. > 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 > http://zugangzureinsicht.org/html/lib/authors/thanissaro/gettingmessage_en.html Three questions: **- What determines an animal's sentience?** **- If killing is never allowed, how would one deal with a parasite infesting their body, or any other animal as so?** **- Would not killing the parasite mean it is self-mortification, & thus an extreme, breaking the Middle Way (as it can hinder the practice of the Dhamma too)?** Please help! Metta to all!