How so? We don't have to create a system in which it is legal to shoot someone in the head and then claim they asked you to do it. Have it be a medical procedure that must be done in a hospital by government certified doctors. You can even require a psych evaluation before it becomes an option.
I can understand there being some gray area in which it isn't clear that the person has all their faculties to the point they can request it, but that shouldn't stop us from creating a system that works for all the obvious cases.
