Because his hands kept moving. You can’t leave a half dead guy next to a gun that he clearly convinced everyone in the place was real. The robber said he was going to shoot everyone. Pointing a gone at someone is battery. You saw everyone pull out their money. They would not have done that if everyone did not think the threat was real. Bodies keep twitching. They don’t die right away. But the gun is right there. And by the way. When a guy comes in and tells everyone he is going to shoot them, you are in a high stress state. And that is a risk the robber took.