If there's a robbery and you somehow apprehend the robbers should you let the hostages out or make them wait?

For example,there's a robbery in a store,they gather all the hostages in a corner.You manage to somehow steal a robbers weapon and say if the rest won't drop their guns you'll shoot his buddy's head off.If they agree i obviously gather them in a corner and call the police(if you didn't already do it before grabbing the gun),do you let the hostages go or wait for the police to come?I wouldn't want to be the guy with the gun that says that everyone needs to stay because the customers are probably scared and they want to get out faster...But there's also a possibility that the robbers have an accomplice among the customers,so it wouldn't be a good idea to let them go.So what should you do?And don't say put the gun down and don't even think of doing something so dangerous =] I'd prefer if a professional in this sort of thing answered.