The idea is that reasoning adults (not like the kid who was cruel for kicks) decide they can't take proper care of the animal and it would suffer or be a harm if left to its own devices. So they put it to as painless a death as possible, which especially if the methods are crude means as quickly as possible. Good grief I hope you aren't comparing injuring an animal for kicks, to managing the livestock and beasts on a farm.
Lots of adults maim and kill lots of animals, for various - and generally unexamined reasons. Hunters, fisherman, farmers, butchers, cooks, auto drivers, SPCA, vets, product test technicians, vivisectionists, etc.
I think that to make an 8 year old bear the focus of adult guilt about mistreating animals is deplorable. Maybe he can't yet distinguish between the live lobster we toss into boiling water in joyful anticipation of a great meal, and using a cat in any way to cheer himself up.