Her behaviour was obviously out of line, but if that was her assigned seat, the real culprit was the person who put the Marine in it. (Perhaps she was in the back making sure a relative or colleague was seated comfortably).
I'm sure none of us would be as rude as she was, but how many of us would casually give up our front-cabin seat (paid for with points, certs, or cash) because someone else was told they could have it?
=aw