[yoo-thuh-ney-zhuh, -zhee-uh, -zee-uh]
Also called mercy killing. the act of putting to death painlessly or allowing to die, as by withholding extreme medical measures, a person or animal suffering from an incurable, especially a painful, disease or condition.
Well, the actual word itself means good death. A good death for non-Klingons would ideally involve no pain. The best way we know how to achieve that is through lethal injection. Mercy killing is the word's extended meaning, as it relieves pain already being suffered. I don't know how good a death it is to have one's skull crushed by Groot, but it's not the first image the word euthanasia evokes.