There's a difference between being OK with it, and understanding why a person didn't intervene. My responsibility is to my wife and my family. Not being there for them is a failure of that responsibility. I didn't sign up to be everybody's hero.
Now, in the case of Julie, the guy confronted her as she was going to her car, and shot her three times. At what point do you not intervene? When you see a knife? When you see a gun? Do you still intervene when weapons are present?
My guess is, most of the people are just playing the vigilante role, when in all actuality, they'd keep their distance. And probably none of these people have ever actually BEEN in a situation that calls for it, so they don't have - but it just looks better to say that they'd be a hero.