The idea of "Stand Your Ground" should only be applied to your home, not public, in public you have no right OR duty to defend anywhere or anything other than yourself, your car, and any dependents you may have. If no shots are fired and no gun is aimed at you stand down, seek retreat and if retreat is not an option then consider the gun, trying to use your gun to help the situation is a bad idea and if you kill the person you could face charges. Also "Stand your ground" laws vary from state to state so before relying on that defence check into the actual laws AND it's use in previous cases. One thing that is happening in Saint Louis area is now the idea of castle doctrine defence is being investigated in all cases rather than just accepted based on the word of the home owner. Also, the shooting in O'Fallon, MO has brought up an important question, in the case of castle doctrine for a car could the man have simply tried driving away from the angered motorcyclist before it turned to punching and then shooting, luckily no charges were filed but a decent case of excessive force could be brought up.
Back up a second here. The law does not require the threat of a firearm. It does require the threat or use of violence. Violence can come in any form, to include physical as we saw with the biker case where the grandfather shot the biker that punched him in the face while inside his own vehicle.
The situation will dictate if any withdrawl is even remotely possible. A human can cover 21 feet in less than 3 seconds. At that distance, and in some cases even farther out, retreat is not an option. A retreat is merely a communication of an intent to withdraw or an intent of no desire to be engaged the situation. If one conditions to always retreat, when no retreat is possible that attempt will still be made. It's a perplexing problem to say the least.
Threats are communicated in two primary ways: Visual and Verbal. The exceptions will be with some form of disability limiting vision or hearing, at that point they will obviously be physical.
No case of excessive force exists with the biker case. Biker committed a felony assault, grandfather acted in accordance with the law. Though some of us may not agree with how the grandfather responded or may think we may have acted differently, none of us were there and cannot say for certain how we would react. He followed the 3 A's.
Access to a reliable firearm.
Ability to use it (physical and training)
Attitude to use it (I will pull the trigger)
Could he have driven away? Possibly, but Missouri law says you have NO duty to retreat from your vehicle. Driving away would be a retreat. That is back-pedalling and we don't want to go backwards.
Next, if you're found justified you cannot face charges, nor can you face a civil lawsuit. Missouri law provides protections against both and prescribes an absolute defense for the justified use of force.
The castle doctrine is supposed to be investigated. "The court shall rule as a matter of law whether the claimed facts and circumstances would, if established, constitute a justification." (RSMO 563.026) I don't see where that involves any kind of "assumption" or lack of an investigation.
http://www.moga.mo.gov/statutes/C500-599/5630000026.HTM