There are circumstances where it can be done legally, but, not many, and they vary from state to state. In New Hampshire, if a dog is experiencing 'undue suffering and pain' and the owner refuses to euthanize, the vet can contact the local authorities and have them authorize the procedure. read more
A veterinary anywhere in the country however, can put an animal to sleep, with or without the owners consent, if the dog is suffering beyond what can be treated with medication. They will most definitely be charged with a crime, and they will certainly face a board. read more