Given the plethora of video camera's that we have in this day and age and the quality of pictures that they can take, you are taking a risk, by not calling the police.
Before someone says, "but they aren't in alleys" think again, business and landowners are putting camera's everywhere, in part to satisfy residents and insurance companies and for thier own piece of mind.
A camera is impartial, but the person looking at the footage can come to the wrong conclusion about what happened and then he/she/they call the police and since they have footage of you, pointing a gun at someone and the innocent person (remember his weapon may not be visible to the camera) turns and runs away and then you holster and go the opposite direction looking around in a furitive manner (perception of the person looking at the film), looking for witnesses to your (il)legal act (again based on the perception of the viewer).
Now the police get a report from you, saying that you had an attempted robbery and how you dealt with it and then this tape shows up and it will be looked at in a totally different light.
I am going to call and report, even though I might have to jump thru some hoops, because of it.
If I don't report and the BG does, then I have to refute his story and since I didn't report right away, who is going to be looked at with a jaundiced eye?