If you haven't read about Ingrid Loyau-Kennett's heroism in London the other day, you should. Immediately after the brutal murder of British miltary drummer Lee Rigby, she hopped off a city bus and talked to the killers while they stood there holding their blood-drenched weapons. It took 20 minutes for the police to show up, at which point they shot and wounded both assailants. In the meantime, there were no other injuries.
Could an armed bystander have saved Rigby's life? It's possible. An armed bystander also might well have made the situation much, much worse. In any case, Loyau-Kennett confronted the killers nonviolently, and no one else was hurt.