I read an article about a delusional criminal and kept waiting for a hero to show up. I never saw one.
Now, I've been called lots of names because I carry a gun every day, but I'm not a vigilante. Intervening in that situation was just plain stupid. No, criminal. And it could have been much worse. There's the story of the guy who heard a woman screaming rape, so he came and shot her assailant. Turned out she was a prostitute and the guy he shot was an undercover cop.
I will defend myself and my family. Intervening when you have no idea who is the good guy and who is the bad guy is a good way to end up in jail or dead. For no good reason:
http://www.stoppingpower.net/commentary/comm_dangers_in_intervention.asp
http://www.thetruthaboutguns.com/2010/09/robert-farago/dont-shoot-three-reasons-why-you-shouldnt-sav-a-stranger/