Not enough details. Is this a situation where the robber was running away and the clerk shot him in the back?
Lots of them just point the gun over their shoulder and fire behind them.
If they have pointed the gun at you and threatened your life, if they still have the gun and are within range of you, they are still deadly threat, running away or not.
It is an invalid legal doctrine to say you have to wait for them to turn and shoot at you to react to the deadly threat which already exists and has already been demonstrated.
There are many cases where people have shot people who are running and deadly threats, and have either not been prosecuted, or won their case, based on self defense doctrine.
I would not care if it was.
The law is utterly failing to protect us. Summary killing of robbers and thieves is the fallback solution. Yes, that solution stinks on ice. A better solution would be to put thieves and robbers in prison for much longer terms when they get caught. They always get caught eventually.
The better solution requires District Attorneys and judges who will follow that path, and replacement of ones who won't follow that path. It also requires money for expansion of prison systems to hold more people for longer terms.
Or we can have vigilantes and "individual initiatives".
What will not continue is government that demands people be disarmed and submit to predators.