Regarding the practice of so-called “mercy killing” or euthanasia, Pope John Paul II in his 1995 encyclical Evangelium Vitae, wrote:

“The choice of euthanasia becomes more serious when it takes the form of a murder committed by others on a person who has in no way requested it and who has never consented to it. The height of arbitrariness and injustice is reached when certain people, such as physicians or legislators, arrogate to themselves the power to decide who ought to live and who ought to die. Once again we find ourselves before the temptation of Eden: to become like God who ‘knows good and evil’ (cf. Gn 3:5). God alone has the power over life and death: ‘It is I who bring both death and life’ (Dt 32:39; cf. 2 K 5:7; 1 S 2:6). But he only exercises this power in accordance with a plan of wisdom and love. When man usurps this power, being enslaved by a foolish and selfish way of thinking, he inevitably uses it for injustice and death. (66.4).