You just can't get your silly little head around it!

It is illegal to target civilians or fire indiscriminately.
Hamas guilty.
It is illegal to fire from civilian area.
Hamas guilty.

If you are fired on by war criminals like that, you may fire back even if civilian casualties are inevitable, but you must give warnings as IDF do, and it must be proportionate to the military importance of the target.

Criminal missile systems and tunnels targeting civilians would be the EXTREMELY important targets.