The fact that the second Marine is sitting down tells us a little about this situation. When being addressed by a senior officer or elected official, it is a matter of USMC courtesy to stand respectfully in their presence. The sitting Marine either does not know that...or he is just dissing Kerry.
The standing Sergeant does indeed look a bit miffed and intimidated. And it is rude for Kerry to be pointing that digit at the Sergeant. He certainly has that "do you know who I am look on his face".
I wish he'd try this on some Marines without control. I think "deck you so fast" might actually be a slight understatement.