The Torah only prohibits melakhot whose purposes are constructive, like the “melekhet maĥshevet” of the Mishkan. Accordingly, one who performs a melakha in a destructive or ruinous manner (derekh kilkul) has not transgressed a Torah prohibition, but only a rabbinic prohibition. This is the meaning of the Sages’ statement: “All who destroy are exempt” (m. Shabbat 13:3). Whenever the term patur (exempt) is used in Masekhet Shabbat it means that one who performs the action is exempt from the punishment mentioned in the Torah, but the act remains forbidden rabbinically (Shabbat 3a).
Thus, one who tears in order to sew transgresses a Torah prohibition, whereas one who tears without the intention of sewing transgresses a rabbinic prohibition (below 13:11); one who dismantles a house or tool in order to refurbish it transgresses a Torah prohibition, while one who dismantles something without intending to rebuild it transgresses a rabbinic prohibition (below 15:1); one who erases letters in order to write others in their place transgresses a Torah prohibition, while one who erases without intent to write transgresses a rabbinic prohibition (below 18:1); one who lights a fire in order to cook, to supply heat or light, or because he needs the ash, transgresses a Torah prohibition, while one who lights a fire to destroy something transgresses a rabbinic prohibition (Shabbat 106a; MT 12:1; below 16:1); one who kills an animal for its meat or skins transgresses a Torah prohibition, while one who kills an animal simply to be destructive transgresses a rabbinic prohibition. So one who steps on ants or kills mosquitoes transgresses a rabbinic prohibition, as there is nothing constructive about this action (below 20:8).