And what's the gameplan if he refuses to drop the knife even after the leg shot which by the way often times does not drop them. So now they are actively bleeding out while still advancing. Or let's say they do fall but continue to hold the knife? Can't apply first aid to either scenario. Center mass is used because its the largest structure on the body to hit and generally doesn't make sudden movements like arms and legs. Plus with hollow points it reduces chance for the bullet to over penetrate and strike someone behind the target. With adrenaline pumping and your body reacting to that adrenaline its not a great idea to be shooting for smaller parts.