I think they us tasers and capsicum sprays and whatnot to neutralize the threat if they're already down?, if they're running at you with a knife then you actually shoot to killactually in Australia they shoot to "neutralize the threat" so if a guy is already down and not getting up again you don't pop him in the back of the head. You pretty much shoot to stop someone from continuing an attack.
The idea of shooting to kill, as opposed to inhibit or injure, is so that you don't abuse the use of a gun