when you shoot a dear and it still breath... do you watch it die helpless or you do what's necessary? what would a lion do?
Though i think it's not about the mercy to give someone a fast painless death. It's more about deaths that might end a life that in fact could be a rich life when death would not have been chosen.
Well, i think, if a person went through a thouroughly psychological examination where several doctors agree that this person will not change his mind or so. If there really is no hope. Then it might make sense. But otherwise society would be at fault ending lives out of a whim.