There are thirty-nine Melachots or prohibited classes of labour on the Shabbat. While the Torah does not define work, the Rabbis determined that work was any act that was required to build the tabernacle during the forty years in the desert.

There are also Toldot which are acts that resemble melacha and are likewise prohibited. The melacha of killing or slaughtering is connected to the melacha that concerns the preparing of skins.

The melacha deals with killing an animal for its skin but it is expanded to include killing or wounding a living being in general for any constructive purpose including for food. This applies to ritual slaughter as well as the slaughtering or killing of any animal, not only kosher animals and is in force regardless of the method used, stabbing, battering or shooting.

In the modern world, this law would seemingly be of little consequence to most of us if it were not expanded to the killing or wounding of any animal or insect.

Killing a bee or mosquito is prohibited accordingly. One is prohibited to kill the bee or mosquito even if it is an annoyance and likely to bite a person.

This would also include a prohibition on swatting a fly or mosquito as it would result in bruising. One may not swat or bruise a person or animal.

There is an exception to the rule against killing if there is a real danger of death. One is therefore permitted to kill a poisonous snake as one is allowed to violate a law of Shabbat in order to save a life.

One may not flush a living fly or other insect down a toilet or drain. Similarly one may not remove a live fish from water.
Interestingly, rabbinic authorities have been cited that indicate that lice may be killed.

It is agreed that it is permissible to spray insect repellant may be sprayed in the air and/or on the body as this acts to repel insects and does not kill them. Placing poison where it would result in the death of an insect or animal is also prohibited even though no blood is drawn.