You are right, but wrong. Florida law does allow the use of force if the verbal threat is one of imminent harm. And threat is longer valid when the the danger has passed. There was no weapon found and he shot the victims while they were in the car. So even if there was threat, with the lack of a weapon, the verbal threat in this case does not present itself as imminent danger.
He is going to go to prison just like the pharmacist who shot the robber again, even though he wasn't moving. Zimmerman may walk free, but this guy is surely going up the river, unless some evidence comes along that supports a weapon was in that vehicle.