The problem with the shooting on the back scenario is that if the guy is wearing a bullet proof vest, reasonably muscular and/or high on something he might just turn around and blast you away, the head is a small target and a lot harder than we think, the legs are way even smaller.
A guy going though the trouble of getting a vest to go shoot up a public place is perhaps as uncommon an ocurrence as the one of a person deciding to out and shoot up a public place. Anything is possible, I hate to find out about the vest after I commit!
They tell armored car guards in training to shoot perps in the triangle formed among the hips and crotch, it is a pretty big reliable target surrounded with "good" realstate to hit even if misssed and gettng shot there will stop most anyone it its tracks, don't know if this will be as effective from the back but have a feeling it is, even a broken femur will do the job.
I suppose that in most other cases were a BG is armed and a treath, any type of armed robery for instance letting them take the money and go is the best policy, it is just material stuff, money can be made again, life once is gone, it is gone forever. You are not getting paid to guard the property of others and even if it is your stuff most probably it is insured but the scenario of some loony already shooting up regular folks just because the neighbor's cockroaches told him to do so is a diferent one, like it would be some terrorist shooting up a shopping mall as in a novel I read recently.
Getting involved then if you are up to it would be more called for in my opinion: there's a big problem and you're the only one with the tool to fix it? If that is the case and if you are not going to make things worse I believe you should. If I had the cojones to get up and do so I think that the thing to do would be to get as close as posible in stealth to fire that first shot, if I can get the gun's muzzle to touch him before pulling the trigger better, if I get caugh midway I am that much closer already and the chances of landing effective shots will be larger than from my original position. Also up close a knife can be a great weapon when you run out of ammo and I always carry a very sharp one.