Everyone has a right to live until they can chose whether they want to continue or whether they are in too much pain. Then I think it should be purely their decision. Euphenasia, I think, is perfectly acceptable, for someone who has a severe condition. As soon as people realize it's not just mindless killing and it can be done in controlled enviroments, it will be accepted.
The choice should never be made by someone else. I don't think that is fair, that is, in my opinion, murder. Unless a decision was made, with the subject, then it's unfair.
You can put an animal down, because it can never make that decision. A vet will only put an animal down if he or she believes that they are certain the animal will die and will be in agony.