"Citizen #5135: In 2015, spit on sidewalks 28x this year, jaywalked 49x, etc..."
Don't worry. Google and the CIA have your back.
/duck and covers from incoming thought police
You had to live -- did live, from habit that became instinct -- in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized. -George Orwell, 1984.
Or in the simple context of computer vision and the camera, raise warnings as individuals on screen act increasingly more suspicious. I imagine future technology will at least have intelligent security cameras that signal alarms based on the current video feed, not just when it's suddenly shut off for example.
Eh, no. Safe preventive action against a gun (or other lethal weapon, really) needs to happen before the weapon is ready for use. What if the assumed offender was wearing protection? Perfect aim wouldn't help. Lethal force might be possible to deploy - but probably not in a safe manner.