> some guy pulls a gun on you, and a video feeds identifies his action, and immediately shoots a tranquilizer straight into his jugular with perfect aim.

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.