Maybe we should change the wording a little in that murder involves grievious bodily harm against another. So committing grievious bodily harm against oneself, would be the same thing. How can we tell other people, or even our family, not to murder other people, and then do something like this to ourselves, without a very solid justification for it?
I have total compassion with someone who has gone so far down the road in illness, such as terminal cancer, that they would like an out. If there were not any legislation to assist the person, then I am almost certain there are many other ways to give the appearance of natural causes. It is not honest, but probably the most humane all round. I'm almost certain many doctors would assist in prescribing medication, and turn the other way when they know that the cancer patient has overmedicated, or used one medication with another with predictable results. I.e. morphine taken with certain heart medication can lead to heart failure. If it is done subtly, and the person is that sick, I can't see a doctor asking for an autopsy. The person would have died from natural causes. As long as the doctor is not asked to give advice in the matter of course.
I think it should be compulsory for all people to have a "living" will, indicating their preference, if they should loose their ability to make a choice whether to live or die when they are in comas, or giving authority to someone to do it on their behalf.