Yes right, if israel is unable to fight conventional war, without massive disproportional amount of civil casualties, they should not probably engage in one.
Civilian/combatant fatality ratios of 3/1 are not uncommon in urban warfare.
Even then, military necessity of killing combatants, not actively particpating in the war is not justified. Yes, you can kill those, who is shooting the rockets at Tel-Aviv, but bombing willy-nilly all of Gaza just because there might be tunnels, where potential combatants might be hiding,. is not acceptable, although not am not an international law lawer.