Shoĥet refers to the melakha of taking the life of a living being. For the Mishkan, they slaughtered teĥashim and goats in order to use their skins for the curtains (Shabbat 73a, 75a).
It is not only slaughtering that is prohibited. Rather, killing a living being in any fashion is forbidden by Torah law. It does not matter whether death is brought about by striking, strangling, or any other method. One who kills a tiny ant violates Torah law, as does one who removes a fish from water, since this kills the fish. Similarly, one who reaches his hand into an animal’s womb and aborts its fetus on Shabbat violates Torah law (Shabbat 107b).
The Torah prohibition applies when the animal is killed for its corpse, i.e., to make use of its meat, skin, or blood. In contrast, one who kills destructively, such as stamping on ants because he wants them dead, transgresses rabbinically.
If one is walking and comes across ants, he should jump over them in order to avoid killing them and violating a rabbinic prohibition. If there is a colony of ants in his path and it is impossible to step there without killing ants, he should walk around them. If he is in a place where he cannot pass without walking on them, he may continue walking, since he does not intend to kill them. It is preferable that he walk on the sides of his feet and do his best to avoid killing ants.
Similarly, if there are insects in a toilet and there is a reasonable chance that flushing the toilet will kill them, it is preferable, when possible, to wait until they fly or crawl away. However, if they do not emerge, or if one must flush for the sake of human dignity, he may do so.
If ants are in a sink and can be blown out of the way, that is best. If it is difficult to do so, dishes or hands may still be washed, even though this will probably drown the ants. Since the one washing is not interested in killing them, and he needs the water, it is not prohibited.
. Even if it is clear that the ants or other insects will be killed, it is a psik reisha de-lo niĥa lei in a case of a double rabbinic prohibition, as the act is destructive and is being done with a shinui. We saw above (ch. 9 n. 2) that when necessary we are lenient in such cases. Even if one insists that the killing here is not done with a shinui, it is still a case of psik reisha de-lo niĥa lei applying to a rabbinic prohibition. In cases of true necessity, we are lenient even in such a case (MB 316:5; SHT 321:68, 337:10). Some maintain that we may be lenient in such a case even le-khatĥila (Yeĥaveh Da’at 2:46). If one walks on the sides of his feet, then the killing is definitely done with a shinui. Besides, it is possible that this will not kill them. Even though Menuĥat Ahava 3:18:10 and Orĥot Shabbat 14:27 are stringent, the primary position is the lenient one, as is recorded in Minĥat Ish 19:9. For a toilet infested with gnats, Minĥat Yitzĥak 10:27 offers additional factors to permit flushing: human dignity, grama, melakha she-eina tzerikha le-gufah, and maybe even mitasek. Shevet Ha–Levi 6:94 is in agreement.Killing lice: Shabbat 12a records a disagreement about killing lice on Shabbat. The conclusion reached there is that killing them involves no prohibition, as they spontaneously generate from inanimate matter. Now, however, since we know that lice do reproduce, it is forbidden to kill them. Halakha is determined in accordance with current knowledge. See Harĥavot.