I have been faced with the decision of ending the life of my ten day old child. He was born without a trachea, and while he was intubated directly into his lungs through a tracheostomy, he would not have lived much longer. The doctors said that there were no heroic measures that could be used to prolong his life, but had I wanted, we could have continued to give him nourishment and oxygen and he could have lived a little longer. He was given a very large dose of morphine regularly by this point, likely because of pain, (newborns have a disproportionately higher pain response than we do as we age) and his life held little of quality that he could get. So... did I let the doctors prolong his life because life is sacred, or did we take him off the oxygen and allow nature to take it's course?
We of course allowed him to die, cradled in my arms, (the first I was allowed even to hold him). He was given enough morphine to remain comfortable, and he died when his body gave in. Not being able to get oxygen into his lungs, it would have really been suffocating, and it was the hardest thing I've done, but had the doctors been able to euthanize him I really do not know what route I would have taken, and when would have been the right time to do so. And knowing how desperately precious life was at this point, who was I to decide for another that they could take theirs? Allowing someone to die, is not the same as helping them to.
With a loved one, you need to take into account their comfort. Can you keep them comfortable to allow them to die peacefully? (Note, I did not say with dignity, there is little that is dignified about much of the human condition, birth and death among them) Can you provide pain control for them? Can you keep them calm? Those are important things.
As to a person who can foresee the future, and their own death... I have read stories in which they will sometimes kill themselves to spare themselves the indignities, to spare themselves the pain. I cannot judge them for this, however, knowing that I would have given anything for my child to live, I do despair of those who end their own lives, and I find it selfish. There is no reason in the world that can justify killing yourself, that is more justifiable than another. All it takes is for them to justify that it's okay for THEM, and to them, it is.
My son's father killed himself several years after this, and even though for him, his life was an emotional torment, not a physical one, who's to say that one torment is any less of a torment than another?? Why should it be okay for a sick, demented old person to kill themself, but not for a sick, mentally ill person? If suicide is allowed to be okay for one reason, how do you determine where to draw the line? How?