Your priorities are out of alignment.
So like a fool, I started looking for his (nonexistent) lens. It took me a few moments to realize that this fifth youth was a plant, meant to distract anyone who came across the mugging.
That's when I called the police. Would I have done things a bit differently, in hindsight? Probably. But in situations like this one, you don't have a whole lot of time to realize exactly what's going on.