Thucy,
(Note: You use the word patient, forget that. The term is only and ever human being)
This is what I have come to understand through being with other's in their life and death struggles and from living through a few of my own.
First I must give you my respect, as I see in you courage. You are asking so many hard questions to which there are few definitive answers. I think, the best answers to life and death questions are usually private and personal, and for which in our individual context we must come to know only for ourselves. Society creates laws about these things to both help us contain what understanding we have and to keep us from making rash decisions; these laws change in the course of new information, we hope...
The person isn't asking to be killed, no one would ask for that -- well, probably one who is being tortured day after day after day... They are asking to be released from a suffering worse than death, knowing that the only release from this suffering is death, which is an inevitable reality of life. I know this is very hard to imagine, it is supposed to be; our mind--survival mechanism numero uno--protects us from such awareness.
With the advance of human science, terminal Illness is measurable. The one with the death sentence will fight it, likely, hopefully. But as every good general knows, there is a moment of realization when it is clear the battle is lost. I don't think that to continue to fight to only extend the suffering is something we have a right to choose for another.
I assure you that when death is staring you in the face, often our choices can be seen quite clearly (even if not , as vamos points out, "straight"), even when we are loaded with pain killers.
We don't know if the human being who is suffering has more to contribute, they, however, do. I assure you, the will to live is powerfully strong. Consider this bit of information, for those who are old and whose bodies are failing them, they will hang on to see a baby born, a couple married, or one last Christmas, and they die almost predictably soon there after; once they know that the reason they hung on a bit longer is safe, they let go, the terminally ill can be different in this respect as it is not necessarily a total body failur but a leathal failur of a bodily system that puts them in so much suffering.
I think that for one to realize for themselves that it is time to die is an experience too profound to be proven to anyone. The body and mind know more together than our combined consciousness has yet found a way to express. I call it instinct.
Having lived through the experience of life threatening illness and all the pain and suffering that goes with it, from my own perspective, I have learned that our instincts will tell us if/when we can let go, as we fight to survive, if we can, and as we feel all the things we have learned about the preciousness of life; as we find every fear we have ever felt about "hell" and damnation and nothingness, and as we list every failure and every success in the hopes that our tallies come out at least balanced. I am of the belief that our innate natures inform us when to let go and to trust that our suffering of life is no longer of value. We all die, and as I wrote on the second abortion thread, I think choosing (and allowing this choice) to let go when life is no longer being lived even in, especially in, a state of futile suffering is a affirmation of life
We can never know or judge the right or wrong reasons someone chooses to end their suffering, it is not ours to assume and I think in our reason we hates this idea as it's as far as we can know for another.
Euthanasia is practiced daily, quietly and hopefully compassionately. Doctors will spike the morphine, deny a diuretic..., when they see only suffering. With human beings in the end stages of life doctors will do the only thing they can do to end immense and unnecessary suffering, they will end the life, sometimes with the human being's knowledge and consent, sometime without it.
Doctor's hasten death this way because our laws have yet to see that terminal suffering is cruel torture, and because we have yet to legislate according to this wisdom (in North America at east) our rights to freely choose our end. I expect that if laws allowed for assisted suicide, the doctors would freely give up this unspoken responsiblity, until then it is the only humane choice.
What our laws will continue to do, is prevent the slippery slope that is feared, if only because the human population will never agree to legalize the practice of euthanasia, though hopefully we will one day see the compassion in allowing the terminally ill human being to chose when to end their own suffering.
I think that to believe that by giving human beings the (and here's that word again) legal choice to end their already dying life would only result in a slippery slope downwards into our basest and cruelest human aspects, is to discount our reverence for life and compassion most have for our fellow human beings. Our insight into other's contexts, personal, racial, sexual and cultural, has done much for our social progress in the last 100 years.
To deny human being their choice over life and death, when ever possible, is fear talking, even shouting, in an attempt drown out our reason and compassion, which says we're all going to die sometime, and fear is never the best thing to listen to when considering our life and death decisions.