It already is legal in the USSA (United Socialist States of America). I see it everyday in the medical profession. Don't kid yourself. If you don't want it to happen to you when you are incapacitated and unable to make your own decisions, you better have an ironclad legal document signed before it is too late. Otherwise, you better hope your next-of-kin likes you. That being said, I think it is humane to end needless suffering.