That was my question. Overkill, yes. And probably should be punished. Premeditated first degree murder? I doubt that the pharmacist was “plotting” to kill the SOB who tried to rob him.
Overkill is the correct response when you are outnumbered AT LEAST 4 to 1 by armed robbers. And unless you are Wyatt Earp, you do not know which outlaws are carrying which weapons or where they are concealed.