You have your answer in your own post. Unless you are a commissioned police officer don't fire on any vehicle in your scenario. This is where many CCW holders have gone wrong in the past. The law is clear, you must prove to a jury of your peers that you or someone near you were in immanent life threatening danger. AND then like you stated, you will be subject to those secondary considerations like maybe the driver is hypoglycemic or having a stroke or seizure. When the risk is too great, one must deescalate. Good topic of discussion though.I mean, obviously, you have determined that something is seriously wrong, but your life isn't directly threatened nor is the life of any of your relatives.
And you really can't determine if the carnage will continue or might be the result of a medical emergency or a problem with the vehicle.