On the surface it would appear that Israel is determined to destroy Hamas regardless of the cost of Palestinian lives and world opinion. This may well be the case, but it's possible that Israel prefers greatly weakening it militarily, but not politically. Why would this be?
Israel actually played a role in the creation of Hamas in the 1970s by funding its Islamist precursors. It did so in an attempt to weaken Arafat's secular Fatah by creating a rival for Palestinian loyalty. At a later point, once Hamas emerged, Likudists thought that the presence of an irredentist Hamas could be used as an excuse to avoid negotiations. Israel would claim it couldn't enter into peace talks because the Palestinians, or a critical segment of them, would never accept the very existence of the state of Israel.
Back in late June of 2006 there were indications that Hamas and Fatah might come to an agreement on a set of principles which would include an implict recognition of Israel within its 1967 borders. Hamas' interest was pragmatic. It needed Western aid for Gaza. There were splits within Hamas on this issue---accepting Israel's existence. But it seemed something was afoot that could have represented a distinctive break from the past.
It was around this time that a wing of Hamas kidnapped an Israeli soldier, Corporal Gilad Shilat. From news accounts it seemed that one motivation was the military wing of Hamas (stationed in Syria) trying to undercut the possible concessions the political leadership in Gaza was contemplating, but it was unclear at the time. Another motive was to induce a prisoner swap.
What is clear is that the Israeli government totally dismissed the possible evolution of Hamas. The next thing that happened was an assault on Gaza and declarations that Israel would never engage in prisoner swaps, despite the fact that it had been done in the past---and would be done in the future. (Israel had, in fasct, swapped large number of prisoners for the dead bodies of a few of its soldiers).
Perhaps Israel was concerned about a "peace scare." The phenomenon has been common in history. President G.W. Bush, for example, was worried inspectors would find no WMD and invaded Iraq to preclude such a development which would have undercut his rationale for war.
Before that, President Nixon worried about a peace treaty with the North Vietnamese coming too soon and hurting his re-election chances in 1972. He was plagued by Churchill being defeated after WWII, because once England had won the Brits wanted to focus on domestic reform and Churchill was a reactionary. Nixon felt he would have trouble winning re-election if foreign policy wasn't at the top of voter's priorities.
Surprisingly,the Munich agreement between Hitler and Chamberlain was perceived by Hitler as a disastrous realization of a "peace scare." Although we now view Chamberlain as a fool, Hitler felt he was the fool for allowing Chamberlain to postpone his war-making. The next time he vowed not to let it happen and made sure that the Poles had no time to accede to German "demands" before invading in 1939.
In the current I/P conflict,if Hamas might accept Israel within its 1967 borders Israel would have to dismantle settlements or engage in land swaps etc. Israeli governments, apart from rhetoric, have never really wanted to do this. In part, the reluctance was a desire to keep what they had ocupied; in part,a fear of internecine warfare with the growing number of settlers.
I recently spoke with an Israeli ex-pat now living in New York, who was a prominent television journalist and later worked for Rabin. He said that of all the Prime Ministers since 1948 only Rabin was serious about peace with the Palestinians and he got assassinated. To him this meant that no future PM would seriously entertain an ambitious dismantling of West Bank settlements---or major land swaps, which would be necessary for peace. On the contrary, whatever they have said they always allowed more settlements to be built.
If this analysis is correct Israel won't truly want to eliminate Hamas as the government force in Gaza. If they did this and Abbas's Fatah became the de facto leaders---something Gazans might well prefer to being slaughtered by the Israeli military---Israel might be put in an awkward position. Better for them to emasculate Hamas as a fighting machine, but keep them espousing irredentist rhetoric in perpetuity.