Israel has been killing Palestinians, expelling them from their homes, and oppressing them for decades before Hamas even existed.
"Both sides" is like saying the Native Americans were at fault for resisting the European Colonists' (ultimately successful) efforts to take their land and exterminate them.
What did you expect? Israel has been expanding more and more into Palestine, murdering and displacing Palestinians who live there. Did you really think nobody would fight back?
Hamas presents new charter accepting a Palestine based on 1967 borders (2017)