I don’t agree with everything David Benatar writes, but I do think he’s right about antinatalism and issues related to it, such as the right to die. In this article, he defends the right to die and euthanasia against common objections.
With that scenario in mind, we can see the hidden assumption in the slippery slope argument against legalizing euthanasia: It is the assumption that the instances of euthanasia that the Netherlands now permits are morally wrong. But the problem is that very many defenders of a legal right to die would deny that those instances of euthanasia are wrong. Some of us think that the suffering that a person endures need not be the product of a terminal disease in order for it to be intolerable. We also recognize that some mental suffering is intractable and as unbearable as physical suffering. And we recognize that it is not only competent patients, but also incompetent ones who can suffer from conditions that make their lives not worth living. Accordingly, we would like to see euthanasia and assisted suicide permitted in such a wider range of cases. If, however, we cannot effect that legal change in one step, we recommend, in the first instance, a more limited liberalization of the law. Once that change has been made, people might realize that the next step and then the next are also acceptable, even if they cannot see it now.
And thanks to Michael in the comments for posting this quote from another article:
The decision about whether to continue living in such conditions is among the most important that can be made. Just as people value having control over where to live, which occupation to pursue, whom to marry, and whether to have children, so people value having control over whether to continue living when quality of life deteriorates. That is why the right to life and the right to die are not two rights, but two aspects or descriptions of the same right. The right to life is the right to decide whether one will OR WILL NOT continue living. The right to die is the right to decide whether one will die (when one could continue living). If the right to life were only a right to decide to continue living and did not also include a right to decide not to continue living, then it would be a DUTY to live rather than a RIGHT to life. The idea that there is a duty to continue living, regardless of how bad life has become, is an implausible one indeed.