I always wonder what a potential hero would do, once convinced the BG's gun was fake and while making the attempt to detain him, if the BG told him to "screw off - are going to shoot me?" as he starts to run away. NOW what do you do? A little embarassing I think, maybe a little dangerous too?
And you KNOW his gun is fake, so shooting him now would be a little hard to explain. Do you tackle/wrestle him? Where is your CCW? Reholstered? In your hand? And now he KNOWS YOU have a gun. Does he want it? Are going to roll around with him?