Tail gator:
Being the third party in that situation, yes, of course, it could be a bad guy in the car, it could be a bad guy outside, they could both be libyan nuclear materials smugglers.
Being the third party puts an armed citizen in a bind. In that situation I could have just covered my own keister and let the chips fall. That's not what I do, though.
When something like this happens, as has happened a couple times, as soon as I hear raised voices, I go on alert, and try to read the situation.
In this situation, visual and language cues probably would have ruled out a narcotics bust on the guy in the van. When a weapon was pulled out without a badge, or shouted warnings, it would be a pretty strong clue that the guy outside the van wasn't a cop.
Lacking any evidence that the guy outside the van had a legitimate reason to drag out and beat the van driver, or god forbid, shoot him in the head, I'd be stuck in the situation of making a shoot/don't shoot decision based only on a few seconds of eyewitness input. I'm also in the equally bad situation of deciding whether or not I should even intervene, and then, stuck with the question of how to do it safely if I do.
The situation for an armed third party would be horrible, and no matter what, that third party will be hung out to dry for intervening and maybe killing or injuring one either D1 or D2. Vigilantes get no respect, and that is what I would be viewed as.
Being 3 cars behind an enclosed van, I don't know how many people are in there, it may even be a van full of nuns transporting mentally disabled orphans to the petting zoo, and D2 was a homicidally angry victim of abuse by nuns.
I'm not someone who can automatically lay down a blanket statement that "it's not my problem. Let them all die, I have a wife/family/self to take care of."
I'm going to see both of those people, D1 and D2 as possibly being the best friends I ever had in grade school, the doctor who saved my neighbor, or so forth. heck, D2 might even simply be a mentally retarded adult that escaped his handlers and is playing "die hard" in the parking lot. I'm going to weigh every bit of information, and be the "professional witness" that an armed person must be.
Am I going to jump out and shoot? heck no. I've been in this situation before, and will again, and will not even expose my weapon until I am absolutely certain that a victim is about to have his life ended or irreversibly altered.
The one thing I can say with certainty, is that if I had witnessed that situation, I'd have had 911 speed dialed before D2 got abreast of the window. It would be very apparent that there was at least a probable assault and battery in progress, and being only 6 feet 190 pounds, I'm not going to try breaking it up myself.
That is one of the reasons that I have carried pepper spray quite often myself. macing some fool is a lot smarter than shooting him, if there is any doubt of the intentions.
If D2 had confronted me in that manner and I saw no weapon, I'd have left my gun still stashed and my pepper spray ready instead.
I can always shoot him in the head while he rolls on the ground gouging at his eyes if I change my mind, right?