I have pretty firm criteria for getting involved in ANY scenario.
There has to be an attacker, actively attacking someone I know he shouldn't be attacking. Which means I have to KNOW who is good and who is bad. You are in a shopping mall and a guy starts shooting mom and the kids in the food court, that one is a no brainer. You are in a classroom and a guy walks in and starts shooting at students and the instructor, no brainer. You are driving down the road and see a cop getting his ass kicked or shot at, no brainer.
Anything short of that? No. I mean seriously, I gotta see someone DYING or on their way to it, before I am intervening. It's too risky otherwise. I have witnessed domestics, I called the cops, no need for me to get involved beyond that. I have also known people who got trapped by their sheepdog mentality. One guy I know got beaten and mugged, a girl was screaming for help, he ran to her and her partner ambushed him. I gotta see the attacker and victim and see it going down, before I am involved.
The situation as described is an excellent scenario in which the average citizen is best off minding their own business.
We may lose and we may win, but we will never be here again.......
Some said "unless you have a badge", negative, EVEN if you have a badge, off-duty this is not the time to jump in.
Call it in, stay on the phone........
I know of one case where the good guy jumped in and ended up shooting the male half. Both the bad guy and the female half claimed that the off-duty started the whole thing, was out of control, pulled the gun first, etc.
In the given scenario I'd have called in the domestic and stayed on the phone, likely been close by observing and shot the dude when the box cutter came out.
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Police are on call, paid, and have back up for these situations. The citizen does not. Unless I see large amounts of blood being drawn, I am going to: 1) call 911, 2) observe, 3) intervene if someone is dying or the guy starts attacking other people with a weapon.
This is the same protocol for all arguments, fights etc. I don't know the background story. I really don't know who the bad guy is or is not--even if the 'victim' is female. Intervention may become option 1 above if the victim is an infant or toddler.
FWIW I have been in this exact situation and did exactly as I described. Even then it was awkward and no fun having the bad guy staring a hole through my head when I'm in a parking lot telling the police he was going MMA on a woman. now the guy knows me, where I live, etc...
My prevailing philosophy on intervening starts from the position that adults can take reasonable measures for self protection. OC, knife, baton, firearm...that they choose not to is NOT my problem. Perhaps the situation they find themselves in will teach them a few things about protection and selecting a better class of associates. Furthermore, the hyper-litigious nature of virtually everyone in the US these days assures me that unless there is a killing spree taking place in front of me, I'm not doing anything besides calling the PD. They have 100x the legal resources that I do should someone get mad that wife-beater is now dead.
One's firm criteria should be based on state statute for use of deadly force. Laws governing use of force aren't as black and white as the media and other groups portray them.
Most businesses, a lot of public areas, and more and more homes are equipped with cameras. Imagine the video if one were to jump into Paul's scenario before the smackdown. Man and woman appear to be having a discussion (no audio) and all of a sudden someone leaves their car and pump, walks purposefully toward them, and pulls a weapon. Then the pair tells the cops, "this guy came at us with a gun..."
"Gunfighting is a thinking man's game. So we might want to bring thinking back into it."-MDFA
Beware of my temper, and the dog that I've found...
In this scenario, mind your own business.
"No free man shall ever be debarred the use of arms." - Thomas Jefferson, Virginia Constitution, Draft 1, 1776