In this case there was enough evidence in advance to do something about it. The school knew (the district will be sued big time), the students knew and the FBI was made aware of this nut case and his social media. He couldn't carry a backpack on campus because they were worried about this.
It doesn't matter ,there are enough evidence everywhere , guns are out of control in this country , I bet you know someone that should not own a gun , and you aren't doing shit about it, I personally know at least 5 people from work , that shouldn't own a gun and if one these fuckers decide to kill bunch of people tomorrow I wouldn't be surprised