In real life, if you were in the military, and walked into an enemy encampment without orders to do so, not in your uniform, and slaughtered everyone inside, that would be counted as murder. I think.
As someone that was in the military, yes. If you kill unarmed enemy soldiers when you are not under orders, being fired upon, etc. it counts as murder. However - see below.
That depends. Could you be considered a spy? Keep in mind that Ulfric gives the player orders to cause as much grief to the imperials as possible. By taking out an entire enemy encampment, he is simply acting upon those orders with great efficiency.
Yes. My character would be closest in real world equivalents to a spy or a special commando unit. I have orders to hurt the enemy and their capacity for war through sabotage and any other way I can discover while behind enemy lines. This is all at my discretion, though in real world terms if captured by the enemy government my own government would deny they were aware of me or my actions.
My character has orders. These soldiers are not town guards. They are not simply Imperials going about their business in a town. They are enemy soldiers in the process of manning and defending a strategy military installation in preparation for continued war actions against my side. Saying it is illegal to kill them is simply wrong. Immoral to slit their throats as they sleep? Yeah, most definitely, but waging war is far removed from moral niceties, especially when you are the underdogs facing a much larger and better equipped enemy.