I suspect you have a concealed handgun license, if so you surely were told that the only justification for "deadly force" is in defense of your life or another persons.
SO let say you get a clear shot and you talk it, bad guy goes down and everybody is happy. Now comes the detectives and one of the first people they talk to is the clerk who was being robbed. NOW your case hang on what the clerk says. IF they say "he was about to shoot me" all is well, BUT what if they say "all this guy wanted was a few bucks for Christmas and he never had any intention of shooting me, I was not afraid and know he would be on his way, I'm not even sure he had a real gun", guess what happens now. Life as you know it is over.