Is there anything in the proportionality principle that can rationally justify killing of any kind?
The motives of vengeance, which drive us to kill those who have killed people we love, are completely irrational, even if we try to wrap them in rational packaging. We exact vengeance because we hate and are hurting, not because we excel in mathematics and logic. Early in the aerial bombing of Gaza, five young girls from the same family were killed, and many more children have died on both sides of the border in recent years. The attempt to introduce their bodies into an equation that would make their deaths justifiable or comprehensible might be necessary to influence current events, but it is still enraging.
The only equation I can wholeheartedly accept is one whereby zero bodies appear on either side of the equation. And until that time comes, I'll choose outcry and protest that appeal solely to the heart. I shall reserve my appeals to the mind for better times.