There are two ways to kill tamed animals in Valheim, turning on Friendly Fire or equipping the Buthcer's Knife. I use the butcher's knife when I am slaughtering large amounts of tamed animals to restock my reagents for making food recipes, but when I find a tamed animal that is not where it should be: like fallen into a moat without an exit; I turn on Friendly Fire, kill the animal, then turn off Friendly Fire.