It must be a premeditated KILLING.
Robbing a store is a premeditated act.
Shooting the cashier, because he suddenly pulled a gun from under the counter wouldn't be a premeditated KILLING though.
It would be murder--felony murder in GA: the illegal killing of a person during the commission of a crime; the killing does not have to be premeditated.
There are probably people serving life sentences in Georgia, I know that their in Florida, for doing just that [shooting the cashier. It is irrelevant why the cashier was shot]who would beg to differ with you.