What unites Palestinian Arabs is their opposition to Jewish nationalism and a desire to stamp it out - not aspirations for their own state. Murdering Jews is a collective Palestinian Arab effort that requires a collectively sensible response.
Israel's reaction to nearly ten years of shelling Israeli civilian population centers from the Gaza Strip is nothing more than a measured, fair response, designed to effectively terminate armed attacks and more importantly - to prevent its recurrence. All of Israels actions in this regard are supported by international law.
Hamas - a United States designated foreign terrorist organization, by their aggression and initial use of armed force against Israeli civilians and non-combatant Jews in breach of the United Nations Charter, constituted prima facie [Latin: on its face] evidence of an act of aggression - aggression being defined by international law as "the most serious and dangerous form of illegal use of force."
Therefore, the rule of proportionality in this case of continuous aggression, needs to be met by Israeli acts that will induce the wrongdoing aggressor to comply with international obligations. A countermeasure need not be the exact equivalent of the breaching act.
United Nations Resolutions demand of states to combat terrorism and reaffirm their:
unequivocal condemnation of all acts, methods and practices of terrorism as criminal and unjustifiable, regardless of their motivation, in all their forms and manifestations, wherever and by whomever committed. 
Judge Schwebel, the former President of the International Court of Justice is quoted saying:
"In the case of action taken for the specific purpose of halting and repelling an armed attack, this does not mean that the action should be more or less commensurate with the attack."
1 See: UN Resolution 3314 (XXIX). Definition of Aggression.
2 United States Department of State, Draft Articles on State Responsibility, Comments of the Government of the United States of America, March 1, 2001. See: www.state.gov/documents/organization/28993.pdf.
3 See: UNSC 1269, 1368, 1373, 1377