I would think it would be the Officer corps that would be less-likely to turn on the civilian populace.
It depends. Even though they're in the uniform, there are some moonbat officers in the bunch. I sure as hell wouldn't mess with the E1-E3 class, because they're all full of piss and vinegar and liable to shoot anything that moves. The E4-E6 class, NCOs and the passed over officers (O4-O6) are probably the best bet to maintain order for the civilians, as well as the best trained to protect them.
No offense, but anyone with stars can't be trusted, unless they're willing to stand with their CSMs/CMSgts/MCPOs.
Unfortunately, far too many young E1 - E3 troops don't even know what Constitution even says much less what it really means, therefore might be the ones most inclined to support martial law.
The E1 - E3 enlisted may not know what the Constitution says, what it means, or what their obligations are in relation to it, but I'm willing to bet that their NCOs do - and when in doubt, the E1 - E3 is more likely to follow the lead of his Sergeant. That's why seeing that the NCOs that would enforce Martial Law - or any similar decree from "on high", are at about half the number of Officers that would gives me heart.
As a side note, how much you want to bet that this is exactly the reason that President Bush rammed through that treaty with the Canadians and Mexicans, allowing mutual aid in the form of Military personnel in the event of a "national emergency" in any of those three countries? Can't depend on your own military to enforce martial law, so get the military of a neighboring country to "help you".