Is there a logical slippery slope from voluntary to nonvoluntary euthanasia?

John Keown has constructed a logical slippery slope argument from voluntary euthanasia (VAE) to nonvoluntary euthanasia (NVAE). VAE if justified implies that death can be of overall benefit, in which case it should also be facilitated in those who cannot consent (NVAE). Hallvard Lillehammer asserts that Keown's argument rests on a fallacy. However, pace Lillehammer, it can be restated to escape this fallacy. Its validity is confirmed by applying to VAE some well-established general principles of medical decision making. Thus, either VAE and NVAE must be accepted together or, if NVAE is regarded as unacceptable, VAE should also be rejected.