One of the major arguments against the legalization of euthanasia are fears of abuse of said practice against the most vulnerable in society. The most commonly cited example in that context are policies of Nazi Germany, where under the policy of "kinder euthanasia" (child euthanasia) the totalitarian state killed severely mentally and physically handicapped children.
Voluntary euthanasia concerns only patients capable of making decisions for themselves; it does not concern mentally impaired, children or people deemed unwanted by society or government, like criminals and minorities. As such, it cannot be argued that the legalization of euthanasia, which could be only requested by a patient in question, would lead to similar results as Nazi Germany policies - the reality of legal euthanasia in places like the Netherlands and Belgium does not give credence to such fears. Strict procedures and mental reevaluation of a patient can limit such risk to the minimum or even prevent it altogether.
The body of research concerning legal euthanasia in places like the Netherlands is not as clear-cut as proponents of said practice claim it is. According to John Keown, a renowned professor of ethics and leading voice against the legalization, there are reasons to believe that legal euthanasia is abused by physicians. Furthermore, proponents of the legalization cannot guarantee that in the future some government wouldn't be tempted to abuse the practice for its own goals.
[P1] Voluntary euthanasia means a physician-assisted suicide on a wish of a patient. [P2] A patient to make a such a wish must have full mental faculties as confirmed by a trained psychologist to make such wish legitimate. [P3] Therefore, voluntary euthanasia doesn't entail the risk of abuse and concerns only interested parties.
Rejecting the premises
[Rejecting P3] Who should be able to request the legal euthanasia is hard to pinpoint from a legal point of view and as such is open to different interpretations.