They are the ones who are doing 90% of the killing right now.They don't have to. They will do whatever they think fit.
In the meantime we can be sceptical about Hamas's complaints about their own people being brutalised for as long as they're not themselves prepared to do whatever is in their own power to de-escalate the conflict.
I addressed that here:
http://www.urban75.net/vbulletin/showpost.php?p=8576492&postcount=1701
Besides which I don't think Hamas should have been firing rockets in the first place.