Based on your excellent montage, 80% of spree killers could be by the following process:
1) Set up security cameras at all entrances to schools, theaters, universities, etc.
2) Screen digitized faces fed through software designed to screen for people whose eyes give away the fact that they are obvious homicidal maniacs.
3) Have law enforcement visit with all who meet the obvious maniac standard.
Viola.
(Only the Navy Yard shooter looks passably sane, if a little intense. Perhaps this is an old photo.)
Tooo costly.
Merely close all, schools, theaters, etc. and have folks learn and be entertained inside their alligator filled, moat protected castle.
Have food airdropped in and have a work at home job.
Problem solved.