Goodness, just listen to all you delicate, suburban flowers!
Were you aware that it used to be a fine old tradition to feed hunt horses to the hound pack, when it was time for them to die? It still happens every so often, but it's not as much as it used to.
There's nothing wrong with death by bullet; many vets think it's faster and much more humane than lethal injection. It's pretty instantaneous, whereas the 'pink stuff' can take quite awhile to work through an animal's system. I plan to have my next horse euthed by bullet, if I get that choice.
It's better all around not to contaminate a body with chemicals, especially one you're going to bury. Chemicals can leach into the ground water, which will cause all sorts of problems for people and other animals.