Much of the opposition to the legalization of assisted dying is based on the fear that voluntary requests from patients for physician aid-in-dying would soon be expanded to allow for patients to make advance directives for euthanasia upon the meeting of particular physical conditions. It would then be further expanded to decisions for euthanasia being made by surrogates or health care agents. Finally, it would expand to the involuntary euthanasia of incompetent patients whom physicians or others believe no longer have any quality of life. The expansion of patient rights to include the use of advance directives and surrogates for euthanasia might bear some truth as the use of advance directives and surrogate decision making which can result in a patient's death is seen to be valid under certain conditions in several countries. In the USA, for example, court rulings from the Quinlan decision to the Cruzan decision have upheld the authority of advance directives and surrogate decision making when patients have made their philosophical intentions known prior to loss of competence. This indeed is the basis of living will provisions that presently exist.
In essence, the courts might eventually see no difference between "allowing to die" and easing suffering by providing for a patient's death more quickly. In terms of moving to non-voluntary euthanasia, opponents of assisted death point to both the Netherlands and Nazi Germany as examples to be avoided. These arguments, however, ignore the facts. In the Netherlands qualified patients can request and receive either lethal prescriptions or direct euthanasia (lethal injections). Government studies have found that some 1,000 patients a year meet death in such an involuntary manner. What is lost in this argument is that the Dutch make no distinction between passive and active euthanasia in their statistics, and that many of these "life terminating acts without explicit request" (LAWER) result from the withholding or withdrawing of life-sustaining treatment -- a practice that is widespread in America. In additionb, many other such deaths in the Netherlands involved prior discussions between patients and physicians. These LAWER cases have come to public attention only because the Dutch have created a system that allows qualified patients to request and receive assistance, and requires physicians to report them.
Also ignored is the fact that non-voluntary "euthanasia" is common in American hospitals but go unreported as these deaths are often sheltered under the wider umbrella of "double effect." In many other cases, especially those involving terminal sedation -- or what has been termed "slow euthanasia" -- deathy is the end result of high dosage sedation together with the withholding of nutrition and hydration. Obviously, death is the intended result of many such actions. In this way the "slippery slope" already exists. Legaization of aid in dying can actually provide the controls that can do much to make this slope explicit, and bring existing abuses to light.
In terms of Nazi Germany, what opponents seem to forget is that official programs for euthanasia were designed from above with the intention from the very beginning of gradually creating a system of genocide rooted in the concept of racial purity. This was never voluntary nor based on freedom of choice. The true slippery slope in Nazi Germany was the loss of civil liberty and freedoms. The current call for legalized assisted dying is based on freedom of choice! Restricting freedom of choice and enforcing a public health model which protects individuals from harming themselves -- even the terminally ill -- is far more dangerous than an approach which provides guarantees of broad freedoms to every individual, including the freedom to choose the time and manner of one's own death.