The reason Hamas is endorsed by the Palestinians is because Israel-Palestine is an apartheid state where Palestinians are oppressed. Unreported acts of robbery, looting, and murder by armed Israeli civilians (non-soldiers) of Palestinian are daily occurrences. It's not a peaceful coexistence between Israelis and Palestinians as advertised in U.S. media, and Hamas is a reaction to that.
Of course this doesn't justify Hamas's actions in any capacity. But it's not a surprising reaction. That said, Hamas does not have the personnel or resources to wage a war against Israel.