Utilitarianism is an ethical theory positing that the best approach to resolving ethical issues should be the one that seeks to maximize "utility", that is the well-being of sentient agents or at least to minimize their suffering. Voluntary euthanasia is often seen as a resolution to the situation of terminally ill people, whose suffering is often described as senseless and tragic. From a utilitarian perspective, prolonging their suffering is unjustifiable.
In cases where terminal illness is incurable and causes a great amount of pain, there’s no reason to sustain a patient’s life and refuse their wish for an assisted suicide. To force someone to remain alive against their will is to force them to live a life they don’t want, and that is unacceptable and inhumane. If we accept the notion that humanity should strive to limit suffering in its all forms, the denial of the legal euthanasia to terminally ill patients is undeniably cruel. There is nothing to gain by forcing terminally ill patients to live their lives in agony; thanks to the practice of euthanasia we can put an end to their suffering, which ethically is much more preferable.
The value of human life cannot be made negotiable or dependent on the quality of life. If our main goal is to alleviate suffering, then the palliative care and anesthetics are a preferable alternative since they do not entail moral dilemmas that legalization of euthanasia does and do not compromise a position of a physician as a person whose task is to save lives, not to end them.
[P1] Terminal illnesses cause great pain and suffering. [P2] Death is a release from suffering. [P3] Therefore, a person with an illness is best placed to decide if suffering is too great for them to handle.
Rejecting the premises
[Rejecting P2] There are preferable alternatives to the death of a patient, such as palliative care.