Right to die

This is not as simple as you said it. If someone who's very sick, and that person's friend/family decides to kill him/her. How can the authority verify that the person really wanted to die in the beginning?
You can have some kind of set ups like the person dying giving consents, but this system can be abused by people making up false consents, or giving consents at times when they can't think logically (e.g drunk or bipolar people at the most depressed stage)
Further more a person who wants someone killed can launch psychological attacks on that person, making them feeling depressed and tricking them into consent of death.
It would also complicate the trials on homicide where the killer can defend themselves by saying they have the other person's consents.

It would make more sense for people in an advanced stage of a terminal disease, or a vegetable if it's thought there's no chance for a recovery. If a person is perfectly healthy, but yet the wish to die, then as I said before they need help, not death.