Is it illegal to kill an old sick dog yourself?

It depends on the jurisdiction, but in general, it is not illegal to kill an old sick dog yourself if the intent is to relieve the animal from pain or suffering, but It is important to note that it is important to consider the legal and ethical implications of euthanizing an animal. Please consult with a local veterinarian, animal shelter, or animal control officer for more information on the laws and regulations in your area regarding euthanasia.