TBH, as long as the vet was there, in the field ready to inject and go as soon as he was done, before he had even a moment to stiffen up, then I wouldn't have such an issue with it.
Good boy, well done... and gone.
Not what I would choose though. I bet the dog would have been happier with a big, juicy steak.
If the vet wasn't ready and waiting, I bet the dog was in immense pain shortly after.