No one is justifying attacks on civilians, but your assertion that this has absolutely nothing to do with occupation is absurd.
HAMAS is not a static group of guys that have been around for 40 years. They are continuously getting new members and recruits, and guess why people might be driven right into their arms? Maybe you start to hate the people who occupy your land, hold you down as a child and break your legs, and indiscriminately kill civilians and journalists during raids into your home.