At first listen, it seems dumb or illogical to arrest the clerk who had been fired upon by the robber, but seeing as a customer was also hit, the police may chose to round up both men for now, until it is known exactly who shot the innocent bystander. That’s the only way it makes sense to me.
Based on the story, I wonder if the criminal charge relates to the fact the clerk left the scene without summoning medical help for the perp. The clerk says he didn’t know he shot the perp, but police found the perp outside the store and still alive. The perp later died.
I’m guessing the police are saying that the perp could have survived if the clerk had called for help in a timely manner. Instead, the clerk locked the store and left the premises.
Every bullet comes with a lawyer attached.