Sounds like the infamous 29 Palms survey that was given about 20 years ago. Here's hoping that the 75% that said "no", start by shooting the 25% that said "yes"...
I would never, ever, ever, turn my weapon on a civilian. Unless said Civilian had his weapon trained on me first.
The thing is, I'm pretty sure that if it came to the Government turning the Marines loose on it's own citizens, they would try and disguise the action as something else so that the Marines would follow the orders..... telling them it's to keep the peace, or that a particular group has separated from the country and is using terrorist actions against other citizens.... that kind of crap.
And it's hard to say how long it would take before the Marines figured it out.... the control of the flow of information is a powerful thing.