Nikolaas TinbergenThinkers

Blood (n.) spilled ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠is a brake built into the instinct: the sight of the opponent wounded or bleeding drains a fight of its aggression.

Modern war (n.) kills ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠so much partly because weapons strike beyond sight: the trigger of seeing the enemy remains, but the brake of seeing the dead and wounded is gone.