To expand on my previous remarks, since 9/11 it has become open season on attackers.
We’re not talking about a “knife fight” or someone flashing a knife. Consider a crowd such as the one that Loughner approached with the majority of attention focused forward. The assailant steps up behind someone alone at the back of crowd, stealthily stabs an unseen blade into the victim’s lower back, says, “Excuse me” then quickly moves on to repeat the same maneuver. Yes, you can swarm him and bring him down — they did that with the shooter. But he can kill two or perhaps five or six folks before most of the crowd realizes there’s any kind of attack going on. If one or more of the victims begins screaming, that could easily lead to more confusion rather than less.