What can we do?  I would suggest two things that we CAN do.  First, ban high capacity magazines.  Second, ban "bump stocks", and crack down on semi auto to automatic conversion kits.  Without those two items, this guy might still have been able to carry out a mass killing, but not 59 dead and hundreds wounded.  You can argue all you want that "if you make them illegal, only criminals will have them".  I say BS.  There is no reason they need to be so EASILY and WIDELY available.