An examination of the mass killings that you mention reveals that they are generally planned for months if not years. The guns are attained illegally in most cases. Locking your gun in a gun safe, if you have a crazy killer living in your house, is not going to keep that killer from getting a gun. Maybe it won't be your gun, but he will get a gun or guns, or explosives, and do what he plans... unless someone intervenes. By the way, if you're not home, how long does it take to defeat most trigger locks or gun safes with some conventional tools? Minutes for most.