I just don't understand this principle. To me, when one is attacked, there is no "disproportionate force" principle.
I remember seeing cartoons in the '50s and '60s when I was growing up, where an obviously evil menace attacked the good guys; the good guys fought them off, but allowed them to lick their wounds and escape to fight again another day. To me, that's what's wrong with this picture. That scenario should apply only to games of sport where there are rules. In warfare, there should be no rules other than: Don't attack first; but if attacked, then decimate your enemy so that he can never do it again.
Israel should do what it needs to do: eliminate Fatah and Hamas utterly and completely. It took carpet bombing, A-bombs, and horrendous ground and sea battles to bring the German Nazis and Japanese neo-Bushido fascists to utter submission and defeat.