That brings up the very difficult and contested issue of what human dignity is.
What has happened, both in the literature and in the courts, is that the exercise of autonomy has been equated with human dignity, so that if you lose your ability to be autonomous—which by definition you do, if you're incompetent, if you have Alzheimer's disease—then you're regarded as being undignified.
The approach to it of the Dying With Dignity” people is, “We will help you by putting you out of your undignified state”, and of course, that's done through a lethal injection. That's the support for euthanasia.
The other concept of dignity, the one that has underlain most of our ethics and law, is that human dignity is intrinsic to being human and that as long as you're human, you have dignity and must be respected in the way that all humans need to be respected. The danger of not taking up that concept is that then anybody who doesn't fulfill the conditions for being seen as having dignity can be disposed of.
It is then, for example, that you get euthanasia of handicapped newborn babies; you can have euthanasia of children.... Peter Singer, for example, the philosopher at Princeton, believes that parents have a right to decide whether to keep a child with disabilities, up to the age of three years. He also thinks that if they can't relate to other people, they don't deserve the protections of human dignity.
This is a bit of self-advertising, perhaps. but I just got a new book out about six weeks ago called Bird on an Ethics Wire, and the third chapter is about 40 pages on the concept of human dignity.