When a Jew slaughters a kosher animal, it is considered dead, even while it is convulsing. The Sages later prohibited eating meat from a convulsing animal, but the fact remains that it is already considered food, which makes it susceptible to ritual impurity. Should it, while convulsing, touch a dead lizard, it would acquire the ritual impurity of food.
If a Jew slaughters a non-kosher animal, then while it is convulsing, it cannot be eaten by anybody: by a Jew, because it is non-kosher, and by a non-Jew, because slaughter does not make it permitted for him, and he must wait till the animal completely dies. Nevertheless, if a Jew slaughters a non-kosher on behalf of a non-Jew, then, since the Jew's intention is to prepare it as food for the non-Jew, it, too, is susceptible to ritual impurity of foods while it is convulsing. Thus, paradoxically, it cannot be eaten, but it is already considered food. The same would be if a non-Jew slaughters a kosher animal.
If one sodomizes a convulsing animal, he is liable, since it is considered partially alive, as proved by the absence of the impurity of dead meat until it dies.
Art: Simon Van Den Berg - Minding the animals