Eh, to a certain extent I think its okay. Like if someone just broke their leg and theyre in a ton of pain they shouldnt be able too obviously. But if they have a disease disability or such that causes a ton of pain and lasts their whole life. Then I think it would be fair to give them the choice. Once again though you have to think about their mental stableness and their family. Although I wouldn't wanna be the one with the job who has to put down the patients who request this...
I think if someone wants to be killed because of too much pain in a broken leg something else might be wrong..
I don't know why NOT to allow someone to be put out of their misery if they consent to it, or their immediate family can, and the costs keeping them alive would be more than the euthanization, unless like abka said if they wanted to just off themselves for a broken leg.