The tranquilizing agent would have to instantly immobilize a wide range of people of different body sizes, levels of intoxication, atypical drug reactions, etc without overdosing others. This problem has been worked on for at least 50 years. Most police officers carry less-than-lethal weapons already (Taser, pepper spray, “bean bag” shotgun rounds), but in the face of an armed attacker there is often no substitute for the stopping power of a bullet.