Coup de grâce

A coup de grâce ( 'blow of mercy') is a death blow to end the suffering of a severely wounded person or animal. It may be a mercy killing of mortally wounded civilians or soldiers, friends or enemies, with or without the sufferer's consent. The meaning has extended to refer to the final event that causes a figurative death.

Modern law
Today, a coup de grâce for incapacitated soldiers would be a war crime: the laws of war mandate caring for the incapacitated and prohibit mercy killing.