I have a theory, and while it's just Lima Theory therefore subject to much error, it is still a theory in need of being proven or disproven.
My theory is that, like B52's fraternity beating up the biggest guy in a bar fight, or your bank robbers going in and taking out the security guards first, etc, it has more to do with "crowd control" than anything else.
Part of pulling off a successful coup, whether it be to get respect, fear or money is to convince the crowd that you are a force best not reckoned with. How do you do this? Well, you go after the person/persons that people may look to for protection and/or guidance. With that person/persons gone you have instant fear and therefore crowd control.
Now, that doesn't necessarily mean the guy with the gun is going to automatically "get it" mostly because he's not expected to be there.
For instance, I'm planning a bank robbery (hypothetically, of course) and I know there is security (armed or not, it doesn't matter). The first order of business is to take them out and gain the fear and control of the crowd. Second order of business is to get the money and get out of there.
People in high stress situations (yes, even criminals) experience tunnel vision. They have an objective, they go for that objective and they sometimes don't even see things they aren't worried about.
Okay, so, I'm robbing a bank. My first objective is the security. I run in screaming for everyone to get down on the ground while I run for the guys with the white shirts and pretty belts. I knock them out and look around.. No one is standing. They are scared crapless and on the ground. Good. I have crowd control. Onto objective #2, the money. I run to the manger and/or tellers and start my withdrawal.
I'm not looking for weapons on the civilians in the bank because that would take too much precious time. Time I don't have to waste, and as I'm experiencing tunnel vision (very likely) there could very well be a guy open carrying right to my left who just happens to be on the floor cowering like everyone else and as long as he doesn't do anything to alert my attention (like get up) I might as well not even know he exists.
Now, let's step that down a little further to a simple convenience store robbery.
If you have watched any of those videos on YouTube, or anywhere else for that matter, and you watched the bad guys closely you can almost SEE their tunnel vision.
They come running in, go right to the counter, do their business and get out of there as quickly as possible.
Often, there are other customers standing around, in a daze, that it doesn't even seem the bad guy even SEES and it very well could be that they never do see them. The customers are just standing still, scared, not knowing what to do and hoping they don't get hurt and it's over before their brain even kicks in that there was something going on.
One of them could be EASILY open carrying a firearm and the bad guy could be none the wiser and therefore there is literally no threat to the open carrier.
Now, let's take it even a step further and talk about random killing like in a mall shooting scenario. These shootings are random and the shooter (from what I've read and seen) doesn't give a crap whether the person is young, old, black, white, security or civilian, armed or walking their dog. His objective is simple, kill as many as possible, and it doesn't make you any more of a target to be open carrying because EVERYONE is already an equal target and, again, he may not even see that you are armed. All he sees is a person, a target, someone to take out. You are just as good as the next victim and therefore you get an equal share in his lunacy, just like everyone else.
I have open carried quite a few times and it always amazes me how many people DON'T see that I'm armed. They can be standing in line right behind me and have absolutely no clue. Why should someone running into a store to rob it have any more awareness and ability to spot my firearm than the guy behind me in line? Just because they are committing a crime, does that make them superman and a weapons expert? Does that give them a sudden ability to home in on firearm? I don't think so.
Now, if I'm open carrying I'm not going to be an idiot, stand up, raise my hand and say, "Ahh, Mr. Burglar. I have a gun." I'm probably going to try to conceal the fact that I am armed, even if it's just turning my gun side away from the action to lower the chance that I am seen and targeted because, though unlikely, is certainly is a possibility and that possibility should not be ignored by people who do decide to open carry.
And even if they don't kill me they can certainly demand my disarmament and try to take my beautiful gun that I don't want to give up. So, in the interest of self preservation, I'm certainly not going to make my status as a carrier any more well known.
Now, to address the, "You've got a gun, why don't you do something?" problem.....
Again, unlikely. First, that would take the civilians around you actually seeing you have a gun, and as I've said, I've open carried before and the vast majority don't even see it.
If they do and that question is voiced, one, was it spoken loud enough for the bad guy to hear it? Two, are they going to press the issue and make themselves a target by drawing unnecessary attention their way? And three, can you knock them out and shut them up? (I'm really kidding on that last point!

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Seriously though, the question might come up but a simple, "SHHHHH!" might be all that's needed to get them to put their heads back down and shut up. They don't want to draw any more attention to themselves than you want to draw attention to yourself. If they panic and start screaming, well, then that's a whole other ball game to play and you have fun with that.
So, in my theory's conclusion, I'm going to say that whether you are a bad guy or a good guy you are still human... right?
I've been around guns all my life, carrying guns for years and working with guns for the last two years. I help people find holsters to conceal, I help people find guns, I write articles on concealing guns and body types and best ways to conceal. I LOOK for guns when I'm out in public and guess what.. I'VE EVEN MISSED A FEW.
I'll be sitting next to a guy for ten minutes, he'll get up to leave and JD will say, "That was a nice Sig he was carrying."
I'll feel like a complete idiot because I completely missed it. I could tell you his approximate hight, weight, skin and hair color and a brief description of his clothes, but somehow I missed the open carried gun, either because of the angle or my eyes just skipped over it.
I'm not perfect and neither are you and neither are cops and neither are FBI agents and neither are TSA guys or anyone else for that matter, and that means the bad guys too.
Chances are DARNED HIGH that they are going to skip, not just your gun, but your ENTIRE PERSON as long as you aren't their objective or in the way of that objective.
But that's all Lima theory.