Hamas

It's rather unuseful to write them off as "crazy," even though they are. The quandary of the 21st century, I believe, will be how to deal with the "crazies" as technology allows them a disproportionate amount of power and access whether it be to WMD or Facebook.

The first step is to figure out why they are shelling Israel.

Possible Reason #1 - They are reacting to some specific Israeli aggression not accurately reported.

Possible Reason #2 - They are acting at the behest of Iran (or the Muslim Brotherhood or Syria or some other client) to test the limit of Israeli patience and military capabilities as a follow up to the Lebanon battle last year.

Possible Reason #3 - They are trying to provoke an Israeli response to distract from their own domestic political problems.

Possible Reason #4 - It is their "reason d entre" or reason to exist, they will shell or otherwise attack Israel for the same reason individuals eat and breathe and procreate.

Possible Reason #5 - They are testing the resolve of Western countries to continue to support the Israeli state by provoking Israel into "disproportionate" response.

I suspect the closest answer is a combination of #3 and #4.