The bottom line is that you'd better get it right and you'd better be able to take full control of the situation and all the people in it if you're going to intervene. It is definitely possible to make things worse than they already are and also to get yourself into a lot of trouble.
I mentioned seeing this before in force on force scenarios and I wanted to springboard off of John’s comment to fill out my remarks above and add another example. In this case these students were all people who had proficient marksmanship and had gone through firearms courses before, though not force on force training.
One of the scenarios was an open air cafe. Two actors enter the scene and harass, punch, and knock two customers at the entrance to the ground. The assailants stop as soon as the victims fall to the ground. The assailants proceed to enter the cafe, knock things off tables, verbally abuse some customers, push people, and then eventually they leave. Out of the twelve students in the class, 10 of the students either blended into the back of the restaurant or waited until they had a clear path and bolted out of the restaurant. Two did not. One student saw the two people being attacked, ran to the assailants and commanded them to stop, and when they turned to him (not moved to him but turned) he shot and incapacitated both. The other student decided to draw her pistol when the assailants got near her in the restaurant. At this point the assailants were between her and the exit (the cafe had a fence around it). She made some initial verbal commands. The assailants ucontinued to get closer to her, taunting her, then eventually one pushed her and she shot him. She stayed in place as the second assailant rendered aid to his friend. In the above the first student was a competition level shooter and the second student was a NRA instructor. I mention this not to disparage those groups, but to point out that even students of some experience can make decisions that are questionable in hindsight.
In the above scenario, students choosing to take action had no impact on the well-being of the initial two victims. Between the available sight lines and the speed of the initial attack there was no way to stop the attackers before they made their first attack. The victims were hurt, though their wounds were presumably not overly serious. The only difference then was what happened to the assailants. In the case of most students the assailants left, hopefully to be reported by police afterwards. In the case of those two students at least one or both of the assailants died. While what can start as simple assaults can turn deadly, shooting someone for knocking someone to the ground may well resort in legal charges.
As an aside, if anyone has an opportunity to do force on force training I recommend he/she closely watches what other students do (if you get the opportunity). Decisions that are obvious to one person are not to someone else. I learned a lot in watching others and listening to the instructors give pointers to each student based on their course of action.
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