Proportionality in terms of war casualties is a stupid, meaningless term! The are about 2M folks living in Gaza; about 8M in Israel.
So, if proportionality were observed, Gazans should be permitted to kill 4 Israelis for each Gazan killed; or perhaps a ratio of 7 Israelis for each 2 Gaza casualties, if we don't count the 1.5M Arab Israelis.

The truth is there should be no casualties on either side, but if the Gazans want to continue fighting, then all bets are off. Israel, nor any country, is obliged to let any of its citizens die because because the other side want s to fight. To quote Gen. Geo. Patton, "The object of war is not to die for your country but to make the other guy die for his."