It's best to call PD, but I'll be the first to admit I've let the BG take off.
As an aside, PD was on the scene twice when I had to draw, before the LEO's as a matter of fact, and the BG was able to leave the scene. ( They were active crime scenes and PD had bigger fish to fry.) I was told to just skedaddle. No reports.
Here's something I've learned over the years having a home and a shop in a high crime area, and YMMV. I always strive to be SA. By doing so, I've either 1/2 drawn, or fully drawn ahead of the BG. If he already has a weapon out, it may already be too late for you. You have to recognize the threat before it's too late. I wish I could articulate it better. I guess it comes down to 'street smarts, or something.