Halakhah su Deuteronomio 12:21

Shechita (Ritual slaughtering as dictated in Jewish Law) is a "positive" mitzvah. Shecita is not obligatory (as the case with obligations of Shofar, Succah and Lulav) since if one does not want to eat meat, there is no need to shecht (slaughter) . However, if someone does want to eat meat- there is an obligation to eat the animal only after shechting it, as the verse says, "You shall sacrifice your cattle and sheep like you have been commanded"(Deut. 12:21). Similarly, the shechita of a wild animal (that is kosher) is dictated by Torah law as the verse says regarding a firstborn animal that has a blemish, "...the same way he will eat the deer and the buck" (Deut. 12:22). One learns from this that a wild animal is alike to a domesticated animal in terms of Shechita (See the Sifrei HaMitzvos of the Rambam and the Smag). Furthermore, the shechita of birds is dictated by the Torah since the verse says, "that he will hunt a hunted animal or bird and will pour its blood" (Lev. 17:13). It is concluded from this verse that the pouring of the blood of a bird (which is part of the shechita process of that bird) is the same as that of an animal (see Sifri on Parshas Re'eh). It was taught by Bar Kafra- "This is the set of rules for the animal and the bird and anything that lives in the water" (Lev. 11). 'The bird' has been placed in between 'the animal' and 'anything that lives in the water'...

All the laws of Shechita were told to Moshe Rabbenu orally at Sinai like all of the Oral law, as it is written " you may slaughter any of the cattle or sheep that the LORD gives you, as I have instructed you (Deuteronomy.12.21)" This mitzvah is apart of the oral tradition. It is taught in a baraita: "Rabbi Yehuda HaNasi says: The Torah states: “And you shall slaughter of your herd and of your flock, which the Lord has given you, as I have commanded you” (Deuteronomy 12:21). This verse teaches that Moses was previously commanded about the halakhot of slaughter, even though they are not written explicitly in the Torah. He was commanded about cutting the gullet and about cutting the windpipe, and about the requirement to cut the majority of one siman for a bird, and the majority of two simanim for an animal." (According to Tosfot), Rebbe finds an allusion to this in the word "כאשר". The "Alef" (achad) alludes to one siman in a bird, the "Shin" (Shtayim) alludes to two simanim in an animal, and the "Resh" (Rubo) alludes to the majority of one is like the whole. Furthermore, the word כאשר is the reverse acrostic of רובו של אחד כמוהו the majority of one is like the whole.

… Divine Providence extends to every man individually. But the condition of the individual beings of other living creatures is undoubtedly the same as has been stated by Aristotle. On that account it is allowed, even commanded, to kill animals; we are permitted to use them according to our pleasure…. There is a rule laid down by our Sages that it is directly prohibited in the Torah to cause pain to an animal based on the words: "Wherefore hast thou smitten thine ass?" (Numbers 22:32). But the object of this rule is to make us perfect; that we should not assume cruel habits; and that we should not uselessly cause pain to others; that, on the contrary, we should be prepared to show pity and mercy to all living creatures, except when necessity demands the contrary: "When thy soul longeth to eat flesh" (Deuteronomy 12:20).²⁸The immediately following verse. “… then thou shalt kill of thy herd and of thy flock … and thou shalt eat within thy gates” (Deuteronomy 12:21), serves to sanction ritual slaughter for purposes of food. Since, in context, the reference in Deuteronomy 12:20 is to ritual slaughter, it is clear that Rambam regards even the painless mode of ritual slaughter, when undertaken other than for purposes of food, as forbidden by reason of ẓa‘ar ba‘alei ḥayyim. However, Rambam would certainly regard ritual slaughter undertaken in order to satisfy other legitimate human needs as tantamount to slaughter for purposes of food. Ritual slaughter other than for purposes of food is clearly permitted as evidenced by the statement of the Gemara, Ḥullin 85b, to the effect that R. Ḥiyya slaughtered a bird in the prescribed manner because he sought to use its blood to destroy worms which had infested his flax. See Sefer Ḥasidim (ed. Reuben Margulies), no. 667. Moreover, when the blood of an animal is necessary for some beneficial purpose, the Gemara, Ḥullin 27b and 85b, permits putting an animal to death even by means other than ritual slaughter in order to conserve its blood. The comments of Rashi, Shabbat 75a, s.v. shoḥet, serve to indicate that, under any circumstances, when an animal is killed for human benefit other than for food, it is not necessary to put it to death by means of ritual slaughter. Cf. Rashi, Ḥullin 27b, s.v. ḥayyav le-khasot. We should not kill animals for the purpose of practicing cruelty or for the purpose of sport.²⁹See also Sefer ha-Ḥinnukh, no. 186, who explains that the slaughter of sanctified animals outside of the Temple precincts, even though the act is performed in the ritually prescribed manner, is forbidden because no purpose is served by such slaughter. Sefer ha-Ḥinnukh comments that wanton killing of animals is tantamount to “shedding blood.”