If you want to start killing people, hire a hit man. Should this practice be established, it will end up with Doctors killing anyone whom someone pays them to kill, then saying that the victim requested it, and qualified under the law. I prefer my hitmen to work for the Mob, not at a hospital.
A) You seem to hold doctors in very low esteem, equating them with paid hit men.
B) You refuse to allow a terminal patient the ability to manage his own demise.
I really wouldn't wish upon you the experience of a terminally ill relative begging for relief from pain and the indignities associated with not being allowed to die. (Yes, they revived him even with a written DNR order.)