Euthanasia
Euthanasia, assisted suicide, or mercy killing was the process of killing a person or animal for reasons considered to be merciful, such as to prevent the prolonged suffering of a terminally ill person or animal. 

Terminally ill persons sometimes chose to commit suicide or to be euthanized by means of assisted suicide. 

Although some cultures, such as Humans, considered the practice immoral, others, such as Vulcans, practiced ritual suicide when they reached a certain infirmity with age. The Bolians had the "double effect" principle, in which an action that relieved suffering could be justifiable even if it had the secondary effect of causing death. ()

In the aftermath of Earth's Third World War, Colonel Green infamously euthanized hundreds of thousands of Humans who were suffering the effects of radiation poisoning. ()

In 2154, Malik rationalized his murder of Raakin by calling it an act of mercy. ()

In 2237, the decision was made to euthanize Spock's beloved pet sehlat I-Chaya when he was mortally wounded, rather than let the creature's suffering be prolonged. This was a critical turning point in the young half-Vulcan's life. () 

Doctor Leonard McCoy reluctantly euthanized his father after his father pleaded with him to "release me" from his never-ending agony by turning off his life support equipment. The guilt and anguish the doctor felt for this event was only made worse by the fact that a cure for his father's disease was discovered shortly after the man's death. ()

In 2365, Thadiun Okona facetiously told Jean-Luc Picard that he (Okona) was "not quite yet ready for mercy killing." ()

Klingons who had suffered an injury or wound that precluded their further service to the Empire sometimes euthanized themselves by performing ritual suicide in a ceremony known as the Hegh'bat. In 2368, it was permissible for Starfleet personnel to assist in the Hegh'bat. ()

In 2371, after losing his lungs to the Vidiians and lying in the 's sickbay confined to an isotropic restraint, Neelix surmised that Tom Paris might try to "pull the plug", meaning euthanizing him, and having Kes all to himself. ()

The Vhnori euthanized their own people who had incurable illnesses or diseases in a cenotaph for the purpose of entering the Next Emanation. Sometimes people who their families perceived as burdens were pressured into this. ()

In 2373, Captain Kathryn Janeway imagined that she was suffering from the phage and The Doctor suggested euthanizing her. ()

In 2377, T'Greth requested to be given a warrior's death after being phasered by Paris, but Janeway told him there would be no mercy killings on her bridge. ()

Seven of Nine provided Icheb a mercy killing, at his request, in 2386 after several of his Borg implants had been brutally extracted by surgeons employed by Bjayzl, leaving him mortally wounded and in excruciating pain. ()

Kovich explained to Dr. Hugh Culber that Lieutenant Commander Yor's travel across time and dimensions had left him suffering to the point that his doctors petitioned the Federation for euthanasia. ()

External link 
 

de:Sterbehilfe
Philosophy
Death