Dog Euthanasia - Is it Ethical?

Long long age, so long ago, dog Euthanasia is considered to be ethical for animals that could not be saved. Nowadays it is considered to be unethical even though it is clearly known that you can't save the dog. Do you think it is good for an ailing dog? Euthasia is nothing but putting the dog into death painlessly.

Following are the reasons for dog euthanasia-

1) Incurable diseases such as Transmissible veneral tumor, etc.
2) Dogs in Coma

What is your opinion about dog euthanasia?