I’m not guessing in what I wrote. There are certain orgs and units that have a tendency to be rebellious, and you can and will see dissension there. But they are small. If you think a private in the 2nd MARDIV or a private in a Ranger battalion is going to tell their squad leader to pound sand regardless of what the subject is, you’re out of touch with reality, and have never been in or worked with units.
Shooting on the range with retired dudes, or even people currently from select units, is not the same as being in and seeing what happens day to day- even in such units. Doubly so in the conventional military. There’s no magic here. Critical thinking and and refusing stupid orders isn’t something people just have when it really counts. It has to be taught, fostered and allowed to be used. It is the exact opposite of what basic and military life is structured for. Almost anyone that can think critically is not functional in most of the military. There’s so much ignorant stuff that happens every day, that those types of people either get out, get chaptered out, or in very few cases move to places where individuality is somewhat tolerated.
I was an E5 in the 2nd Mar Div, and most certainly had PFCs and Lance Criminals tell me to "to pound sand" at times. When yelling and more PT didn't work the MPs got involved real quick, but it did happen. There are a LOT of undisciplined troops these days, or at least in my day in the mid 2000's.
Aside from that, I think a certain percentage of troops would follow orders to shoot protestors if such an order was given, but a bigger percentage would also hesitate if they weren't in immediate danger. I honestly don't know what the hell I would have done in that situation, but despite some lack of discipline most all Marines I served with had good enough judgment to decipher whether shooting into a crowd of Americans would've been a lawful order or not, and would have acted accordingly. I suspect most would have refused if they weren't actively being attacked because they are smart enough to decipher lawful orders from unlawful ones; and had balls enough to refuse the unlawful ones if those ever came (which I never saw happen).
