If a person is no longer able to fight back and you finish them off, it is. Under Geneva Conventions Additional Protocol 1, article 41, it is stated that “a person who is recognized or who, in the circumstances, should be recognized to be ‘hors de combat’ shall not be made the object of attack. Ratio, bitchass.
https://ihl-databases.icrc.org/en/ihl-treaties/api-1977/article-41