Eh. Bystander effect is generally shitty, because it means that most people stand by and do nothing while other people do terrible shit. I think a better response is for more responsible people to carry protection items like pepper spray and spray the shit out of people once it becomes clear that a lethal situation is happening or very clearly about to happen.
Also, crazy people ARE more likely to back down in the face of a majority, but two people is generally not enough to scare someone if they're truly off their rocker. You see and hear about incidents where it takes five officers to take a man down (usually mentally ill and/or on some sort of illicit substance), it happens more often then you think.
If you're going to confront someone, have backup.