Euthanasia

This can also occur in fiction with sentiments in direction of "kill me before I become one of Them" or "I want to die as a human and not as one of them", which are often involved with viral creatures such as zombies, werewolves, and such where the euthanasia is done notably before the "suffering" happens, likely because the person is psychologically incapable of handling with the situation and thus is mentally suffering long before the actual thing happens to them.

Note that this is often claimed by some (fictional) murderers that they're helping people be released of their (largely imaginary) suffering. These should NOT be included here as they're murder plain and simple, there is not assisting in it involved. Some cases of euthanasia where the persons concession is not necessary can also be included, such as person who is being eaten alive (by zombies as is common), burning alive, or similar long process of dying that is accompanied by rather ceaseless agony (as long as the person performing the mercy kill isn't responsible for the former agony, of course).

See also: suicide, murder, coup de grâce