An interesting dilemma. In a case like that, where it's clear what's happening (a dude is being chased by a fat rent-a-cop), I'd probably see if I could stop him. My logic goes that if he were truly armed and dangerous, he probably wouldn't be being chased by a clerk.

I've had some similar situations happen here in LA that show it could go either way on giving a hand.

For example, this past Sunday night, I was down in Santa Monica with my girlfriend. We'd just had dinner and were going off to the mall. It was a foggy night. As we were standing on the corner, waiting to cross the street, there were some squealing tires and honking horns. Running out of the fog was a black dude, carrying a shopping bag, being chased by two white dudes. In that area, that could have been multiple scenarios: a homeless guy stole someone's bag, and was being chased; a guy was being robbed by a couple punks; etc. Ultimately, the guy was caught in the middle of the intersection, and was like "here... take it." In that area, that could have been multiple scenarios: a homeless guy stole someone's bag, and was being chased; a guy was being robbed by a couple punks; etc. In the fog, it wasn't obvious what was going on. At that point, I just took hold of my girlfriend's arm, walked her off, and said "we're not getting involved in this." I sure was curious about it, but at that point, it wasn't just my own personal safety I was concerned about. Had I been on my own, I definitely would have stuck around to monitor the situation, and step in if necessary.

Of course, several years of training in mixed-martial arts may be giving me a false sense of bravado -- as a former instructor said at one point during my training, "you have enough knowledge to be a danger to yourself."