I've come across some good advices in books and so forth. The one that pops to mind went something likestay outside behind cover and be a good witness. Included was information to the effect thatentering the scene canhave unpredictable consequences with people getting hurtwho might not otherwise.
My thinking on it has only gone so far as to assume that there isn't anything I can do to help the clerk if the robber already intends to shoot. That is to say, the clerk's fate was sealed when the gun came out.
However, I may be in a position to help the second clerk, customers, or even the first clerk (from a coup-de-grace, if the first shot wasn't fatal). I think taking that first shot if the felon turns, etcwill depend totally on the exact circumstances and whether I have a high degree of certainty of success. I wouldn't distract the felon because I'm not certain that being startled wouldn't trigger a shot. Also, action beats reaction. Mas Ayoob in In the Gravest Extreme seemed pretty confident an armed late-night burglar can turn and fire faster than a homeowner can recognize the increased threat and respond,the homeowner having his gun already up. The situation is a little different, but I personally don't have a high certainty that I can split time fine enough to trigger my shot in that fraction of an instant where he's turned away but hasn't yet triggered his shot at me.If he whirls on me, its going to have all the speed of his already high-adrenalined nerves. It would bea whole new situation if he dives behind cover as his first motion (say he freezes for a moment with the gun on the clerk, and then decides to dive for cover as his first action rather than whirl on me). Read that situation as"gun fight."
Note: Saving the clerk from a coup-de-graceafter the first shot is going to need to be very distinct from vindictive shot.If he stops presenting a threat to the clerk, it can be viewed as no longer having all the elements that justify deadly force.