Remember..the definition of self defense is "The defending of YOURSELF, or OTHERS who would be allowed that level of force". So if the badguy has a gun in the clerks face, which would justify the CLERK using deadly force, you are justified using deadly force. As to the other? Remember the 4 rules..ie.."Be certain of your target and what lays beyond". In your scenario, shooting the badguy in the back, if SAFE would be the ideal situation. Last I checked...bullets work just as well back to front as front to back. Dont let a false sense of chivalry make you think you need to give him a "chance" to engage YOU! He made his choice when he pulled his weapon in a criminal manner. And if more law abiding armed citizens took appropriate and prudent action...