"Collateral damage" and "human shield" are, among other things, both terms employed by belligerents to excuse themselves of responsibility for civilian casualties they caused. But they are not the same excuse.
The idea of "collateral damage" says that we aimed to kill enemy combatants but have ended up accidentally killing hapless civilians, too, who happened to be in a wrong place at a wrong time. The deaths of civilians, for which no one can be held responsible, are to be regretted.
The idea of "human shield," on the other hand, says that it is enemy combatants who caused not only deaths of civilians on our side but those on their own side as well. In the case of Gaza, it goes like this:
Hamas uses despicable tactics -- such as targeting Israeli civilians while using Palestinian civilians as human shields. As a result, the Israeli Defense Forces are forced to combat the terror infrastructure in the midst of the civilian Palestinian population. This leads to pain and suffering not only on the Israeli side, but also to innocent Palestinians, who are forced by means of violence and dictatorship to collaborate with Hamas and its commanders. (Histadrut -- General Federation of Labour in Israel)Israeli civilians killed by Hamas = Hamas's responsibility.
Palestinian civilians killed by Israel = Hamas's responsibility.
In short, Israel is proclaiming to the world: it is irresponsible, less by accident ("We are not responsible for accidental collateral damage") than by nature ("Hamas made us do it, whatever it is. We are helpless. Israel is a victim of Hamas, and so are Palestinian civilians"), and all responsibility belongs to its enemy.
Israel says it wants the world, including Arabs, to recognize its right to exist -- in other words, it wants legitimacy in the eyes of the world and normal relations with other states, especially Arab states. Arabs, however, must wonder: Israel takes no responsibility for the choices it has made -- the choice to occupy other people's territories, the choice to keep building settlements on them, the choice to blockade and impoverish people, the choice to make war to begin with and the choice to bomb schools and other civilian infrastructure in the course of it. Should a state that claims only rights and no responsibility enjoy legitimacy in the eyes of the world? Should the state of irresponsibility be normalized?
And it is not just Arabs who wonder about such questions. In one European Commission survey, taken a little over a year after Israel's "Operation Defensive Shield," during which it reduced Jenin to rubble, nearly 60% of the respondents said: "Israel is the greatest threat to world peace." Imagine how many more will say so after Gaza.