For me, euthanasia is the killing of another person, under conditions that the killer has justified as being in the victim's best interests, i.e. they have no hope of recovery, etc.
Having the victim's pre-written authorization does not make it right in my book. It is still murder.
Sometimes, tho, sick or elderly people stop eating and waste away. I believe that's part of the natural death process.
I do believe however, that an individual can make a decision before hand, if their brain is dead and death is imminent and the only means of life is a ventilator or heart lung machine, that it is okay for that individual to have requested legally beforehand to be denied that means of support.
That is a personal decision that I would not stand against, whether I would choose that or not, and I do not believe it is murder to not provide that level of support in those circumstances if the desire is made known before hand legally. Just my opinion.