If one wishes to eat meat from an animal, he is required to first shecht (slaughter) the animal. According to the Rambam, this is a positive mitzvah as well as a prerequisite to eating meat. However, according to the Raavad, there is no mitzvah per se; if one wishes to eat meat, he must shecht the animal in order to remove the existing prohibitions on eating a live animal.
There are various requirements as to who can perform the shechitah. Optimally one should know the halachos involved and be a male over 13 years of age. If an individual is unfamiliar with the halachos of shechitah he should not shecht, as many problematic issues that he is unaware of can arise. He is also required to be an observant Jew.
One difference between these two requirements is that if one who is familiar with the halachos supervises the shechitah of one who does not know the halachos, and testifies that the shechitah was valid, the shechitah is indeed valid. However, even if one who is familiar with the halachos supervises a non-Jew or non-observant Jew during his shechting, the shechitah is invalid. But there is a dispute as to the source and reason why a non-Jew is unfit to shecht.
Tosafos (Chullin 3b) quotes from a tosefta: this requirement comes from the fact that it is written in Devarim 12:21, “v’zavachta … v’achalta,” alluding to the fact that one who shechts an animal must also abide by the halachos of eating kosher. This is why a non-observant Jew may not shecht an animal.
The Rambam (Hilchos Shechitah 4:11) explains the reason why the shechitah of a non-Jew is invalid: There is a pasuk in this week’ parshah, Parshas Ki Sisa (34:15), that warns the Bnei Yisrael to not form a treaty with the nations of the land – lest they get invited to eat from their slaughter (avodah zarah). We deduce from here that one should not eat from a non-Jew because one will eventually come to eat from his avodah zarah.
The difference between these two opinions is whether we consider the shechitah of a non-Jew a shechitah, or is it simply that the Torah does not want us to eat the meat because it is not considered a valid shechitah? According to Tosafos, the actual shechitah is invalid; it is as if the animal died on its own. An animal that dies on its own or by means other than shechitah is a neveilah, which is forbidden to be eaten and is also metamei (conveying tumah).
According to the Rambam, who opines that the source declaring that a non-Jew cannot shecht is deduced from a pasuk implying that the Torah does not want us to eat from his meat, an animal shechted by a non-Jew is considered shechted – but is forbidden mi’de’oraisa to be eaten. Such an animal will not be metamei mi’de’oraisa, for the Torah only forbade its consumption. Since an actual shechitah was performed, the animal will not convey tumah mi’de’oraisa. (See Rambam’s Hilchos Shar Avos Hatumah 2:10.)
This also has another ramification. Two people may hold onto different sides of a knife while shechting an animal. If a non-Jew and an observant Jew both hold onto the knife, the halacha should vary based on the dispute cited above. According to the Rambam, a non-Jew’s shechitah is considered a shechitah – but the Torah does not want us to eat from his shechitah. Therefore, in this scenario the shechitah will be invalid. According to Tosafos, a non-Jew’s shechitah is not considered a shechitah and it is considered as if he did not shecht. Thus, if he should shecht an animal together with an observant Jew, the shechitah should be valid.