Related Rule
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
The Hellenic Navy’s International Law Manual (1995) provides:
[C]ertain restrictions have been crystallized in both conventional and customary law of armed conflict regarding the employment by the belligerents of means and methods which shall cause to the adversary “superfluous injury or unnecessary suffering”, since a basic precondition of every armed conflict is to render hors de combat the military forces of the enemy without employing means and measures which are unnecessary and incompatible with the notion of reasonableness, even under the harsh conditions of hostilities. 
Greece, International Law Manual, Hellenic Navy General Staff, Directorate A2, Division IV, 1995, Chapter 5, § 2.